btrfs: uninline some static inline helpers from backref.h

[thirdparty/linux.git] / fs / btrfs / backref.c

// SPDX-License-Identifier: GPL-2.0
/*
 * Copyright (C) 2011 STRATO.  All rights reserved.
 */

#include <linux/mm.h>
#include <linux/rbtree.h>
#include <trace/events/btrfs.h>
#include "ctree.h"
#include "disk-io.h"
#include "backref.h"
#include "ulist.h"
#include "transaction.h"
#include "delayed-ref.h"
#include "locking.h"
#include "misc.h"
#include "tree-mod-log.h"
#include "fs.h"
#include "accessors.h"
#include "extent-tree.h"
#include "relocation.h"
#include "tree-checker.h"

/* Just arbitrary numbers so we can be sure one of these happened. */
#define BACKREF_FOUND_SHARED     6
#define BACKREF_FOUND_NOT_SHARED 7

struct extent_inode_elem {
        u64 inum;
        u64 offset;
        u64 num_bytes;
        struct extent_inode_elem *next;
};

static int check_extent_in_eb(struct btrfs_backref_walk_ctx *ctx,
                              const struct btrfs_key *key,
                              const struct extent_buffer *eb,
                              const struct btrfs_file_extent_item *fi,
                              struct extent_inode_elem **eie)
{
        const u64 data_len = btrfs_file_extent_num_bytes(eb, fi);
        u64 offset = key->offset;
        struct extent_inode_elem *e;
        const u64 *root_ids;
        int root_count;
        bool cached;

        if (!ctx->ignore_extent_item_pos &&
            !btrfs_file_extent_compression(eb, fi) &&
            !btrfs_file_extent_encryption(eb, fi) &&
            !btrfs_file_extent_other_encoding(eb, fi)) {
                u64 data_offset;

                data_offset = btrfs_file_extent_offset(eb, fi);

                if (ctx->extent_item_pos < data_offset ||
                    ctx->extent_item_pos >= data_offset + data_len)
                        return 1;
                offset += ctx->extent_item_pos - data_offset;
        }

        if (!ctx->indirect_ref_iterator || !ctx->cache_lookup)
                goto add_inode_elem;

        cached = ctx->cache_lookup(eb->start, ctx->user_ctx, &root_ids,
                                   &root_count);
        if (!cached)
                goto add_inode_elem;

        for (int i = 0; i < root_count; i++) {
                int ret;

                ret = ctx->indirect_ref_iterator(key->objectid, offset,
                                                 data_len, root_ids[i],
                                                 ctx->user_ctx);
                if (ret)
                        return ret;
        }

add_inode_elem:
        e = kmalloc(sizeof(*e), GFP_NOFS);
        if (!e)
                return -ENOMEM;

        e->next = *eie;
        e->inum = key->objectid;
        e->offset = offset;
        e->num_bytes = data_len;
        *eie = e;

        return 0;
}

static void free_inode_elem_list(struct extent_inode_elem *eie)
{
        struct extent_inode_elem *eie_next;

        for (; eie; eie = eie_next) {
                eie_next = eie->next;
                kfree(eie);
        }
}

static int find_extent_in_eb(struct btrfs_backref_walk_ctx *ctx,
                             const struct extent_buffer *eb,
                             struct extent_inode_elem **eie)
{
        u64 disk_byte;
        struct btrfs_key key;
        struct btrfs_file_extent_item *fi;
        int slot;
        int nritems;
        int extent_type;
        int ret;

        /*
         * from the shared data ref, we only have the leaf but we need
         * the key. thus, we must look into all items and see that we
         * find one (some) with a reference to our extent item.
         */
        nritems = btrfs_header_nritems(eb);
        for (slot = 0; slot < nritems; ++slot) {
                btrfs_item_key_to_cpu(eb, &key, slot);
                if (key.type != BTRFS_EXTENT_DATA_KEY)
                        continue;
                fi = btrfs_item_ptr(eb, slot, struct btrfs_file_extent_item);
                extent_type = btrfs_file_extent_type(eb, fi);
                if (extent_type == BTRFS_FILE_EXTENT_INLINE)
                        continue;
                /* don't skip BTRFS_FILE_EXTENT_PREALLOC, we can handle that */
                disk_byte = btrfs_file_extent_disk_bytenr(eb, fi);
                if (disk_byte != ctx->bytenr)
                        continue;

                ret = check_extent_in_eb(ctx, &key, eb, fi, eie);
                if (ret == BTRFS_ITERATE_EXTENT_INODES_STOP || ret < 0)
                        return ret;
        }

        return 0;
}

struct preftree {
        struct rb_root_cached root;
        unsigned int count;
};

#define PREFTREE_INIT   { .root = RB_ROOT_CACHED, .count = 0 }

struct preftrees {
        struct preftree direct;    /* BTRFS_SHARED_[DATA|BLOCK]_REF_KEY */
        struct preftree indirect;  /* BTRFS_[TREE_BLOCK|EXTENT_DATA]_REF_KEY */
        struct preftree indirect_missing_keys;
};

/*
 * Checks for a shared extent during backref search.
 *
 * The share_count tracks prelim_refs (direct and indirect) having a
 * ref->count >0:
 *  - incremented when a ref->count transitions to >0
 *  - decremented when a ref->count transitions to <1
 */
struct share_check {
        struct btrfs_backref_share_check_ctx *ctx;
        struct btrfs_root *root;
        u64 inum;
        u64 data_bytenr;
        u64 data_extent_gen;
        /*
         * Counts number of inodes that refer to an extent (different inodes in
         * the same root or different roots) that we could find. The sharedness
         * check typically stops once this counter gets greater than 1, so it
         * may not reflect the total number of inodes.
         */
        int share_count;
        /*
         * The number of times we found our inode refers to the data extent we
         * are determining the sharedness. In other words, how many file extent
         * items we could find for our inode that point to our target data
         * extent. The value we get here after finishing the extent sharedness
         * check may be smaller than reality, but if it ends up being greater
         * than 1, then we know for sure the inode has multiple file extent
         * items that point to our inode, and we can safely assume it's useful
         * to cache the sharedness check result.
         */
        int self_ref_count;
        bool have_delayed_delete_refs;
};

static inline int extent_is_shared(struct share_check *sc)
{
        return (sc && sc->share_count > 1) ? BACKREF_FOUND_SHARED : 0;
}

static struct kmem_cache *btrfs_prelim_ref_cache;

int __init btrfs_prelim_ref_init(void)
{
        btrfs_prelim_ref_cache = kmem_cache_create("btrfs_prelim_ref",
                                        sizeof(struct prelim_ref),
                                        0,
                                        SLAB_MEM_SPREAD,
                                        NULL);
        if (!btrfs_prelim_ref_cache)
                return -ENOMEM;
        return 0;
}

void __cold btrfs_prelim_ref_exit(void)
{
        kmem_cache_destroy(btrfs_prelim_ref_cache);
}

static void free_pref(struct prelim_ref *ref)
{
        kmem_cache_free(btrfs_prelim_ref_cache, ref);
}

/*
 * Return 0 when both refs are for the same block (and can be merged).
 * A -1 return indicates ref1 is a 'lower' block than ref2, while 1
 * indicates a 'higher' block.
 */
static int prelim_ref_compare(struct prelim_ref *ref1,
                              struct prelim_ref *ref2)
{
        if (ref1->level < ref2->level)
                return -1;
        if (ref1->level > ref2->level)
                return 1;
        if (ref1->root_id < ref2->root_id)
                return -1;
        if (ref1->root_id > ref2->root_id)
                return 1;
        if (ref1->key_for_search.type < ref2->key_for_search.type)
                return -1;
        if (ref1->key_for_search.type > ref2->key_for_search.type)
                return 1;
        if (ref1->key_for_search.objectid < ref2->key_for_search.objectid)
                return -1;
        if (ref1->key_for_search.objectid > ref2->key_for_search.objectid)
                return 1;
        if (ref1->key_for_search.offset < ref2->key_for_search.offset)
                return -1;
        if (ref1->key_for_search.offset > ref2->key_for_search.offset)
                return 1;
        if (ref1->parent < ref2->parent)
                return -1;
        if (ref1->parent > ref2->parent)
                return 1;

        return 0;
}

static void update_share_count(struct share_check *sc, int oldcount,
                               int newcount, struct prelim_ref *newref)
{
        if ((!sc) || (oldcount == 0 && newcount < 1))
                return;

        if (oldcount > 0 && newcount < 1)
                sc->share_count--;
        else if (oldcount < 1 && newcount > 0)
                sc->share_count++;

        if (newref->root_id == sc->root->root_key.objectid &&
            newref->wanted_disk_byte == sc->data_bytenr &&
            newref->key_for_search.objectid == sc->inum)
                sc->self_ref_count += newref->count;
}

/*
 * Add @newref to the @root rbtree, merging identical refs.
 *
 * Callers should assume that newref has been freed after calling.
 */
static void prelim_ref_insert(const struct btrfs_fs_info *fs_info,
                              struct preftree *preftree,
                              struct prelim_ref *newref,
                              struct share_check *sc)
{
        struct rb_root_cached *root;
        struct rb_node **p;
        struct rb_node *parent = NULL;
        struct prelim_ref *ref;
        int result;
        bool leftmost = true;

        root = &preftree->root;
        p = &root->rb_root.rb_node;

        while (*p) {
                parent = *p;
                ref = rb_entry(parent, struct prelim_ref, rbnode);
                result = prelim_ref_compare(ref, newref);
                if (result < 0) {
                        p = &(*p)->rb_left;
                } else if (result > 0) {
                        p = &(*p)->rb_right;
                        leftmost = false;
                } else {
                        /* Identical refs, merge them and free @newref */
                        struct extent_inode_elem *eie = ref->inode_list;

                        while (eie && eie->next)
                                eie = eie->next;

                        if (!eie)
                                ref->inode_list = newref->inode_list;
                        else
                                eie->next = newref->inode_list;
                        trace_btrfs_prelim_ref_merge(fs_info, ref, newref,
                                                     preftree->count);
                        /*
                         * A delayed ref can have newref->count < 0.
                         * The ref->count is updated to follow any
                         * BTRFS_[ADD|DROP]_DELAYED_REF actions.
                         */
                        update_share_count(sc, ref->count,
                                           ref->count + newref->count, newref);
                        ref->count += newref->count;
                        free_pref(newref);
                        return;
                }
        }

        update_share_count(sc, 0, newref->count, newref);
        preftree->count++;
        trace_btrfs_prelim_ref_insert(fs_info, newref, NULL, preftree->count);
        rb_link_node(&newref->rbnode, parent, p);
        rb_insert_color_cached(&newref->rbnode, root, leftmost);
}

/*
 * Release the entire tree.  We don't care about internal consistency so
 * just free everything and then reset the tree root.
 */
static void prelim_release(struct preftree *preftree)
{
        struct prelim_ref *ref, *next_ref;

        rbtree_postorder_for_each_entry_safe(ref, next_ref,
                                             &preftree->root.rb_root, rbnode) {
                free_inode_elem_list(ref->inode_list);
                free_pref(ref);
        }

        preftree->root = RB_ROOT_CACHED;
        preftree->count = 0;
}

/*
 * the rules for all callers of this function are:
 * - obtaining the parent is the goal
 * - if you add a key, you must know that it is a correct key
 * - if you cannot add the parent or a correct key, then we will look into the
 *   block later to set a correct key
 *
 * delayed refs
 * ============
 *        backref type | shared | indirect | shared | indirect
 * information         |   tree |     tree |   data |     data
 * --------------------+--------+----------+--------+----------
 *      parent logical |    y   |     -    |    -   |     -
 *      key to resolve |    -   |     y    |    y   |     y
 *  tree block logical |    -   |     -    |    -   |     -
 *  root for resolving |    y   |     y    |    y   |     y
 *
 * - column 1:       we've the parent -> done
 * - column 2, 3, 4: we use the key to find the parent
 *
 * on disk refs (inline or keyed)
 * ==============================
 *        backref type | shared | indirect | shared | indirect
 * information         |   tree |     tree |   data |     data
 * --------------------+--------+----------+--------+----------
 *      parent logical |    y   |     -    |    y   |     -
 *      key to resolve |    -   |     -    |    -   |     y
 *  tree block logical |    y   |     y    |    y   |     y
 *  root for resolving |    -   |     y    |    y   |     y
 *
 * - column 1, 3: we've the parent -> done
 * - column 2:    we take the first key from the block to find the parent
 *                (see add_missing_keys)
 * - column 4:    we use the key to find the parent
 *
 * additional information that's available but not required to find the parent
 * block might help in merging entries to gain some speed.
 */
static int add_prelim_ref(const struct btrfs_fs_info *fs_info,
                          struct preftree *preftree, u64 root_id,
                          const struct btrfs_key *key, int level, u64 parent,
                          u64 wanted_disk_byte, int count,
                          struct share_check *sc, gfp_t gfp_mask)
{
        struct prelim_ref *ref;

        if (root_id == BTRFS_DATA_RELOC_TREE_OBJECTID)
                return 0;

        ref = kmem_cache_alloc(btrfs_prelim_ref_cache, gfp_mask);
        if (!ref)
                return -ENOMEM;

        ref->root_id = root_id;
        if (key)
                ref->key_for_search = *key;
        else
                memset(&ref->key_for_search, 0, sizeof(ref->key_for_search));

        ref->inode_list = NULL;
        ref->level = level;
        ref->count = count;
        ref->parent = parent;
        ref->wanted_disk_byte = wanted_disk_byte;
        prelim_ref_insert(fs_info, preftree, ref, sc);
        return extent_is_shared(sc);
}

/* direct refs use root == 0, key == NULL */
static int add_direct_ref(const struct btrfs_fs_info *fs_info,
                          struct preftrees *preftrees, int level, u64 parent,
                          u64 wanted_disk_byte, int count,
                          struct share_check *sc, gfp_t gfp_mask)
{
        return add_prelim_ref(fs_info, &preftrees->direct, 0, NULL, level,
                              parent, wanted_disk_byte, count, sc, gfp_mask);
}

/* indirect refs use parent == 0 */
static int add_indirect_ref(const struct btrfs_fs_info *fs_info,
                            struct preftrees *preftrees, u64 root_id,
                            const struct btrfs_key *key, int level,
                            u64 wanted_disk_byte, int count,
                            struct share_check *sc, gfp_t gfp_mask)
{
        struct preftree *tree = &preftrees->indirect;

        if (!key)
                tree = &preftrees->indirect_missing_keys;
        return add_prelim_ref(fs_info, tree, root_id, key, level, 0,
                              wanted_disk_byte, count, sc, gfp_mask);
}

static int is_shared_data_backref(struct preftrees *preftrees, u64 bytenr)
{
        struct rb_node **p = &preftrees->direct.root.rb_root.rb_node;
        struct rb_node *parent = NULL;
        struct prelim_ref *ref = NULL;
        struct prelim_ref target = {};
        int result;

        target.parent = bytenr;

        while (*p) {
                parent = *p;
                ref = rb_entry(parent, struct prelim_ref, rbnode);
                result = prelim_ref_compare(ref, &target);

                if (result < 0)
                        p = &(*p)->rb_left;
                else if (result > 0)
                        p = &(*p)->rb_right;
                else
                        return 1;
        }
        return 0;
}

static int add_all_parents(struct btrfs_backref_walk_ctx *ctx,
                           struct btrfs_root *root, struct btrfs_path *path,
                           struct ulist *parents,
                           struct preftrees *preftrees, struct prelim_ref *ref,
                           int level)
{
        int ret = 0;
        int slot;
        struct extent_buffer *eb;
        struct btrfs_key key;
        struct btrfs_key *key_for_search = &ref->key_for_search;
        struct btrfs_file_extent_item *fi;
        struct extent_inode_elem *eie = NULL, *old = NULL;
        u64 disk_byte;
        u64 wanted_disk_byte = ref->wanted_disk_byte;
        u64 count = 0;
        u64 data_offset;
        u8 type;

        if (level != 0) {
                eb = path->nodes[level];
                ret = ulist_add(parents, eb->start, 0, GFP_NOFS);
                if (ret < 0)
                        return ret;
                return 0;
        }

        /*
         * 1. We normally enter this function with the path already pointing to
         *    the first item to check. But sometimes, we may enter it with
         *    slot == nritems.
         * 2. We are searching for normal backref but bytenr of this leaf
         *    matches shared data backref
         * 3. The leaf owner is not equal to the root we are searching
         *
         * For these cases, go to the next leaf before we continue.
         */
        eb = path->nodes[0];
        if (path->slots[0] >= btrfs_header_nritems(eb) ||
            is_shared_data_backref(preftrees, eb->start) ||
            ref->root_id != btrfs_header_owner(eb)) {
                if (ctx->time_seq == BTRFS_SEQ_LAST)
                        ret = btrfs_next_leaf(root, path);
                else
                        ret = btrfs_next_old_leaf(root, path, ctx->time_seq);
        }

        while (!ret && count < ref->count) {
                eb = path->nodes[0];
                slot = path->slots[0];

                btrfs_item_key_to_cpu(eb, &key, slot);

                if (key.objectid != key_for_search->objectid ||
                    key.type != BTRFS_EXTENT_DATA_KEY)
                        break;

                /*
                 * We are searching for normal backref but bytenr of this leaf
                 * matches shared data backref, OR
                 * the leaf owner is not equal to the root we are searching for
                 */
                if (slot == 0 &&
                    (is_shared_data_backref(preftrees, eb->start) ||
                     ref->root_id != btrfs_header_owner(eb))) {
                        if (ctx->time_seq == BTRFS_SEQ_LAST)
                                ret = btrfs_next_leaf(root, path);
                        else
                                ret = btrfs_next_old_leaf(root, path, ctx->time_seq);
                        continue;
                }
                fi = btrfs_item_ptr(eb, slot, struct btrfs_file_extent_item);
                type = btrfs_file_extent_type(eb, fi);
                if (type == BTRFS_FILE_EXTENT_INLINE)
                        goto next;
                disk_byte = btrfs_file_extent_disk_bytenr(eb, fi);
                data_offset = btrfs_file_extent_offset(eb, fi);

                if (disk_byte == wanted_disk_byte) {
                        eie = NULL;
                        old = NULL;
                        if (ref->key_for_search.offset == key.offset - data_offset)
                                count++;
                        else
                                goto next;
                        if (!ctx->skip_inode_ref_list) {
                                ret = check_extent_in_eb(ctx, &key, eb, fi, &eie);
                                if (ret == BTRFS_ITERATE_EXTENT_INODES_STOP ||
                                    ret < 0)
                                        break;
                        }
                        if (ret > 0)
                                goto next;
                        ret = ulist_add_merge_ptr(parents, eb->start,
                                                  eie, (void **)&old, GFP_NOFS);
                        if (ret < 0)
                                break;
                        if (!ret && !ctx->skip_inode_ref_list) {
                                while (old->next)
                                        old = old->next;
                                old->next = eie;
                        }
                        eie = NULL;
                }
next:
                if (ctx->time_seq == BTRFS_SEQ_LAST)
                        ret = btrfs_next_item(root, path);
                else
                        ret = btrfs_next_old_item(root, path, ctx->time_seq);
        }

        if (ret == BTRFS_ITERATE_EXTENT_INODES_STOP || ret < 0)
                free_inode_elem_list(eie);
        else if (ret > 0)
                ret = 0;

        return ret;
}

/*
 * resolve an indirect backref in the form (root_id, key, level)
 * to a logical address
 */
static int resolve_indirect_ref(struct btrfs_backref_walk_ctx *ctx,
                                struct btrfs_path *path,
                                struct preftrees *preftrees,
                                struct prelim_ref *ref, struct ulist *parents)
{
        struct btrfs_root *root;
        struct extent_buffer *eb;
        int ret = 0;
        int root_level;
        int level = ref->level;
        struct btrfs_key search_key = ref->key_for_search;

        /*
         * If we're search_commit_root we could possibly be holding locks on
         * other tree nodes.  This happens when qgroups does backref walks when
         * adding new delayed refs.  To deal with this we need to look in cache
         * for the root, and if we don't find it then we need to search the
         * tree_root's commit root, thus the btrfs_get_fs_root_commit_root usage
         * here.
         */
        if (path->search_commit_root)
                root = btrfs_get_fs_root_commit_root(ctx->fs_info, path, ref->root_id);
        else
                root = btrfs_get_fs_root(ctx->fs_info, ref->root_id, false);
        if (IS_ERR(root)) {
                ret = PTR_ERR(root);
                goto out_free;
        }

        if (!path->search_commit_root &&
            test_bit(BTRFS_ROOT_DELETING, &root->state)) {
                ret = -ENOENT;
                goto out;
        }

        if (btrfs_is_testing(ctx->fs_info)) {
                ret = -ENOENT;
                goto out;
        }

        if (path->search_commit_root)
                root_level = btrfs_header_level(root->commit_root);
        else if (ctx->time_seq == BTRFS_SEQ_LAST)
                root_level = btrfs_header_level(root->node);
        else
                root_level = btrfs_old_root_level(root, ctx->time_seq);

        if (root_level + 1 == level)
                goto out;

        /*
         * We can often find data backrefs with an offset that is too large
         * (>= LLONG_MAX, maximum allowed file offset) due to underflows when
         * subtracting a file's offset with the data offset of its
         * corresponding extent data item. This can happen for example in the
         * clone ioctl.
         *
         * So if we detect such case we set the search key's offset to zero to
         * make sure we will find the matching file extent item at
         * add_all_parents(), otherwise we will miss it because the offset
         * taken form the backref is much larger then the offset of the file
         * extent item. This can make us scan a very large number of file
         * extent items, but at least it will not make us miss any.
         *
         * This is an ugly workaround for a behaviour that should have never
         * existed, but it does and a fix for the clone ioctl would touch a lot
         * of places, cause backwards incompatibility and would not fix the
         * problem for extents cloned with older kernels.
         */
        if (search_key.type == BTRFS_EXTENT_DATA_KEY &&
            search_key.offset >= LLONG_MAX)
                search_key.offset = 0;
        path->lowest_level = level;
        if (ctx->time_seq == BTRFS_SEQ_LAST)
                ret = btrfs_search_slot(NULL, root, &search_key, path, 0, 0);
        else
                ret = btrfs_search_old_slot(root, &search_key, path, ctx->time_seq);

        btrfs_debug(ctx->fs_info,
                "search slot in root %llu (level %d, ref count %d) returned %d for key (%llu %u %llu)",
                 ref->root_id, level, ref->count, ret,
                 ref->key_for_search.objectid, ref->key_for_search.type,
                 ref->key_for_search.offset);
        if (ret < 0)
                goto out;

        eb = path->nodes[level];
        while (!eb) {
                if (WARN_ON(!level)) {
                        ret = 1;
                        goto out;
                }
                level--;
                eb = path->nodes[level];
        }

        ret = add_all_parents(ctx, root, path, parents, preftrees, ref, level);
out:
        btrfs_put_root(root);
out_free:
        path->lowest_level = 0;
        btrfs_release_path(path);
        return ret;
}

static struct extent_inode_elem *
unode_aux_to_inode_list(struct ulist_node *node)
{
        if (!node)
                return NULL;
        return (struct extent_inode_elem *)(uintptr_t)node->aux;
}

static void free_leaf_list(struct ulist *ulist)
{
        struct ulist_node *node;
        struct ulist_iterator uiter;

        ULIST_ITER_INIT(&uiter);
        while ((node = ulist_next(ulist, &uiter)))
                free_inode_elem_list(unode_aux_to_inode_list(node));

        ulist_free(ulist);
}

/*
 * We maintain three separate rbtrees: one for direct refs, one for
 * indirect refs which have a key, and one for indirect refs which do not
 * have a key. Each tree does merge on insertion.
 *
 * Once all of the references are located, we iterate over the tree of
 * indirect refs with missing keys. An appropriate key is located and
 * the ref is moved onto the tree for indirect refs. After all missing
 * keys are thus located, we iterate over the indirect ref tree, resolve
 * each reference, and then insert the resolved reference onto the
 * direct tree (merging there too).
 *
 * New backrefs (i.e., for parent nodes) are added to the appropriate
 * rbtree as they are encountered. The new backrefs are subsequently
 * resolved as above.
 */
static int resolve_indirect_refs(struct btrfs_backref_walk_ctx *ctx,
                                 struct btrfs_path *path,
                                 struct preftrees *preftrees,
                                 struct share_check *sc)
{
        int err;
        int ret = 0;
        struct ulist *parents;
        struct ulist_node *node;
        struct ulist_iterator uiter;
        struct rb_node *rnode;

        parents = ulist_alloc(GFP_NOFS);
        if (!parents)
                return -ENOMEM;

        /*
         * We could trade memory usage for performance here by iterating
         * the tree, allocating new refs for each insertion, and then
         * freeing the entire indirect tree when we're done.  In some test
         * cases, the tree can grow quite large (~200k objects).
         */
        while ((rnode = rb_first_cached(&preftrees->indirect.root))) {
                struct prelim_ref *ref;

                ref = rb_entry(rnode, struct prelim_ref, rbnode);
                if (WARN(ref->parent,
                         "BUG: direct ref found in indirect tree")) {
                        ret = -EINVAL;
                        goto out;
                }

                rb_erase_cached(&ref->rbnode, &preftrees->indirect.root);
                preftrees->indirect.count--;

                if (ref->count == 0) {
                        free_pref(ref);
                        continue;
                }

                if (sc && ref->root_id != sc->root->root_key.objectid) {
                        free_pref(ref);
                        ret = BACKREF_FOUND_SHARED;
                        goto out;
                }
                err = resolve_indirect_ref(ctx, path, preftrees, ref, parents);
                /*
                 * we can only tolerate ENOENT,otherwise,we should catch error
                 * and return directly.
                 */
                if (err == -ENOENT) {
                        prelim_ref_insert(ctx->fs_info, &preftrees->direct, ref,
                                          NULL);
                        continue;
                } else if (err) {
                        free_pref(ref);
                        ret = err;
                        goto out;
                }

                /* we put the first parent into the ref at hand */
                ULIST_ITER_INIT(&uiter);
                node = ulist_next(parents, &uiter);
                ref->parent = node ? node->val : 0;
                ref->inode_list = unode_aux_to_inode_list(node);

                /* Add a prelim_ref(s) for any other parent(s). */
                while ((node = ulist_next(parents, &uiter))) {
                        struct prelim_ref *new_ref;

                        new_ref = kmem_cache_alloc(btrfs_prelim_ref_cache,
                                                   GFP_NOFS);
                        if (!new_ref) {
                                free_pref(ref);
                                ret = -ENOMEM;
                                goto out;
                        }
                        memcpy(new_ref, ref, sizeof(*ref));
                        new_ref->parent = node->val;
                        new_ref->inode_list = unode_aux_to_inode_list(node);
                        prelim_ref_insert(ctx->fs_info, &preftrees->direct,
                                          new_ref, NULL);
                }

                /*
                 * Now it's a direct ref, put it in the direct tree. We must
                 * do this last because the ref could be merged/freed here.
                 */
                prelim_ref_insert(ctx->fs_info, &preftrees->direct, ref, NULL);

                ulist_reinit(parents);
                cond_resched();
        }
out:
        /*
         * We may have inode lists attached to refs in the parents ulist, so we
         * must free them before freeing the ulist and its refs.
         */
        free_leaf_list(parents);
        return ret;
}

/*
 * read tree blocks and add keys where required.
 */
static int add_missing_keys(struct btrfs_fs_info *fs_info,
                            struct preftrees *preftrees, bool lock)
{
        struct prelim_ref *ref;
        struct extent_buffer *eb;
        struct preftree *tree = &preftrees->indirect_missing_keys;
        struct rb_node *node;

        while ((node = rb_first_cached(&tree->root))) {
                struct btrfs_tree_parent_check check = { 0 };

                ref = rb_entry(node, struct prelim_ref, rbnode);
                rb_erase_cached(node, &tree->root);

                BUG_ON(ref->parent);    /* should not be a direct ref */
                BUG_ON(ref->key_for_search.type);
                BUG_ON(!ref->wanted_disk_byte);

                check.level = ref->level - 1;
                check.owner_root = ref->root_id;

                eb = read_tree_block(fs_info, ref->wanted_disk_byte, &check);
                if (IS_ERR(eb)) {
                        free_pref(ref);
                        return PTR_ERR(eb);
                }
                if (!extent_buffer_uptodate(eb)) {
                        free_pref(ref);
                        free_extent_buffer(eb);
                        return -EIO;
                }

                if (lock)
                        btrfs_tree_read_lock(eb);
                if (btrfs_header_level(eb) == 0)
                        btrfs_item_key_to_cpu(eb, &ref->key_for_search, 0);
                else
                        btrfs_node_key_to_cpu(eb, &ref->key_for_search, 0);
                if (lock)
                        btrfs_tree_read_unlock(eb);
                free_extent_buffer(eb);
                prelim_ref_insert(fs_info, &preftrees->indirect, ref, NULL);
                cond_resched();
        }
        return 0;
}

/*
 * add all currently queued delayed refs from this head whose seq nr is
 * smaller or equal that seq to the list
 */
static int add_delayed_refs(const struct btrfs_fs_info *fs_info,
                            struct btrfs_delayed_ref_head *head, u64 seq,
                            struct preftrees *preftrees, struct share_check *sc)
{
        struct btrfs_delayed_ref_node *node;
        struct btrfs_key key;
        struct rb_node *n;
        int count;
        int ret = 0;

        spin_lock(&head->lock);
        for (n = rb_first_cached(&head->ref_tree); n; n = rb_next(n)) {
                node = rb_entry(n, struct btrfs_delayed_ref_node,
                                ref_node);
                if (node->seq > seq)
                        continue;

                switch (node->action) {
                case BTRFS_ADD_DELAYED_EXTENT:
                case BTRFS_UPDATE_DELAYED_HEAD:
                        WARN_ON(1);
                        continue;
                case BTRFS_ADD_DELAYED_REF:
                        count = node->ref_mod;
                        break;
                case BTRFS_DROP_DELAYED_REF:
                        count = node->ref_mod * -1;
                        break;
                default:
                        BUG();
                }
                switch (node->type) {
                case BTRFS_TREE_BLOCK_REF_KEY: {
                        /* NORMAL INDIRECT METADATA backref */
                        struct btrfs_delayed_tree_ref *ref;
                        struct btrfs_key *key_ptr = NULL;

                        if (head->extent_op && head->extent_op->update_key) {
                                btrfs_disk_key_to_cpu(&key, &head->extent_op->key);
                                key_ptr = &key;
                        }

                        ref = btrfs_delayed_node_to_tree_ref(node);
                        ret = add_indirect_ref(fs_info, preftrees, ref->root,
                                               key_ptr, ref->level + 1,
                                               node->bytenr, count, sc,
                                               GFP_ATOMIC);
                        break;
                }
                case BTRFS_SHARED_BLOCK_REF_KEY: {
                        /* SHARED DIRECT METADATA backref */
                        struct btrfs_delayed_tree_ref *ref;

                        ref = btrfs_delayed_node_to_tree_ref(node);

                        ret = add_direct_ref(fs_info, preftrees, ref->level + 1,
                                             ref->parent, node->bytenr, count,
                                             sc, GFP_ATOMIC);
                        break;
                }
                case BTRFS_EXTENT_DATA_REF_KEY: {
                        /* NORMAL INDIRECT DATA backref */
                        struct btrfs_delayed_data_ref *ref;
                        ref = btrfs_delayed_node_to_data_ref(node);

                        key.objectid = ref->objectid;
                        key.type = BTRFS_EXTENT_DATA_KEY;
                        key.offset = ref->offset;

                        /*
                         * If we have a share check context and a reference for
                         * another inode, we can't exit immediately. This is
                         * because even if this is a BTRFS_ADD_DELAYED_REF
                         * reference we may find next a BTRFS_DROP_DELAYED_REF
                         * which cancels out this ADD reference.
                         *
                         * If this is a DROP reference and there was no previous
                         * ADD reference, then we need to signal that when we
                         * process references from the extent tree (through
                         * add_inline_refs() and add_keyed_refs()), we should
                         * not exit early if we find a reference for another
                         * inode, because one of the delayed DROP references
                         * may cancel that reference in the extent tree.
                         */
                        if (sc && count < 0)
                                sc->have_delayed_delete_refs = true;

                        ret = add_indirect_ref(fs_info, preftrees, ref->root,
                                               &key, 0, node->bytenr, count, sc,
                                               GFP_ATOMIC);
                        break;
                }
                case BTRFS_SHARED_DATA_REF_KEY: {
                        /* SHARED DIRECT FULL backref */
                        struct btrfs_delayed_data_ref *ref;

                        ref = btrfs_delayed_node_to_data_ref(node);

                        ret = add_direct_ref(fs_info, preftrees, 0, ref->parent,
                                             node->bytenr, count, sc,
                                             GFP_ATOMIC);
                        break;
                }
                default:
                        WARN_ON(1);
                }
                /*
                 * We must ignore BACKREF_FOUND_SHARED until all delayed
                 * refs have been checked.
                 */
                if (ret && (ret != BACKREF_FOUND_SHARED))
                        break;
        }
        if (!ret)
                ret = extent_is_shared(sc);

        spin_unlock(&head->lock);
        return ret;
}

/*
 * add all inline backrefs for bytenr to the list
 *
 * Returns 0 on success, <0 on error, or BACKREF_FOUND_SHARED.
 */
static int add_inline_refs(struct btrfs_backref_walk_ctx *ctx,
                           struct btrfs_path *path,
                           int *info_level, struct preftrees *preftrees,
                           struct share_check *sc)
{
        int ret = 0;
        int slot;
        struct extent_buffer *leaf;
        struct btrfs_key key;
        struct btrfs_key found_key;
        unsigned long ptr;
        unsigned long end;
        struct btrfs_extent_item *ei;
        u64 flags;
        u64 item_size;

        /*
         * enumerate all inline refs
         */
        leaf = path->nodes[0];
        slot = path->slots[0];

        item_size = btrfs_item_size(leaf, slot);
        ei = btrfs_item_ptr(leaf, slot, struct btrfs_extent_item);

        if (ctx->check_extent_item) {
                ret = ctx->check_extent_item(ctx->bytenr, ei, leaf, ctx->user_ctx);
                if (ret)
                        return ret;
        }

        flags = btrfs_extent_flags(leaf, ei);
        btrfs_item_key_to_cpu(leaf, &found_key, slot);

        ptr = (unsigned long)(ei + 1);
        end = (unsigned long)ei + item_size;

        if (found_key.type == BTRFS_EXTENT_ITEM_KEY &&
            flags & BTRFS_EXTENT_FLAG_TREE_BLOCK) {
                struct btrfs_tree_block_info *info;

                info = (struct btrfs_tree_block_info *)ptr;
                *info_level = btrfs_tree_block_level(leaf, info);
                ptr += sizeof(struct btrfs_tree_block_info);
                BUG_ON(ptr > end);
        } else if (found_key.type == BTRFS_METADATA_ITEM_KEY) {
                *info_level = found_key.offset;
        } else {
                BUG_ON(!(flags & BTRFS_EXTENT_FLAG_DATA));
        }

        while (ptr < end) {
                struct btrfs_extent_inline_ref *iref;
                u64 offset;
                int type;

                iref = (struct btrfs_extent_inline_ref *)ptr;
                type = btrfs_get_extent_inline_ref_type(leaf, iref,
                                                        BTRFS_REF_TYPE_ANY);
                if (type == BTRFS_REF_TYPE_INVALID)
                        return -EUCLEAN;

                offset = btrfs_extent_inline_ref_offset(leaf, iref);

                switch (type) {
                case BTRFS_SHARED_BLOCK_REF_KEY:
                        ret = add_direct_ref(ctx->fs_info, preftrees,
                                             *info_level + 1, offset,
                                             ctx->bytenr, 1, NULL, GFP_NOFS);
                        break;
                case BTRFS_SHARED_DATA_REF_KEY: {
                        struct btrfs_shared_data_ref *sdref;
                        int count;

                        sdref = (struct btrfs_shared_data_ref *)(iref + 1);
                        count = btrfs_shared_data_ref_count(leaf, sdref);

                        ret = add_direct_ref(ctx->fs_info, preftrees, 0, offset,
                                             ctx->bytenr, count, sc, GFP_NOFS);
                        break;
                }
                case BTRFS_TREE_BLOCK_REF_KEY:
                        ret = add_indirect_ref(ctx->fs_info, preftrees, offset,
                                               NULL, *info_level + 1,
                                               ctx->bytenr, 1, NULL, GFP_NOFS);
                        break;
                case BTRFS_EXTENT_DATA_REF_KEY: {
                        struct btrfs_extent_data_ref *dref;
                        int count;
                        u64 root;

                        dref = (struct btrfs_extent_data_ref *)(&iref->offset);
                        count = btrfs_extent_data_ref_count(leaf, dref);
                        key.objectid = btrfs_extent_data_ref_objectid(leaf,
                                                                      dref);
                        key.type = BTRFS_EXTENT_DATA_KEY;
                        key.offset = btrfs_extent_data_ref_offset(leaf, dref);

                        if (sc && key.objectid != sc->inum &&
                            !sc->have_delayed_delete_refs) {
                                ret = BACKREF_FOUND_SHARED;
                                break;
                        }

                        root = btrfs_extent_data_ref_root(leaf, dref);

                        if (!ctx->skip_data_ref ||
                            !ctx->skip_data_ref(root, key.objectid, key.offset,
                                                ctx->user_ctx))
                                ret = add_indirect_ref(ctx->fs_info, preftrees,
                                                       root, &key, 0, ctx->bytenr,
                                                       count, sc, GFP_NOFS);
                        break;
                }
                case BTRFS_EXTENT_OWNER_REF_KEY:
                        ASSERT(btrfs_fs_incompat(ctx->fs_info, SIMPLE_QUOTA));
                        break;
                default:
                        WARN_ON(1);
                }
                if (ret)
                        return ret;
                ptr += btrfs_extent_inline_ref_size(type);
        }

        return 0;
}

/*
 * add all non-inline backrefs for bytenr to the list
 *
 * Returns 0 on success, <0 on error, or BACKREF_FOUND_SHARED.
 */
static int add_keyed_refs(struct btrfs_backref_walk_ctx *ctx,
                          struct btrfs_root *extent_root,
                          struct btrfs_path *path,
                          int info_level, struct preftrees *preftrees,
                          struct share_check *sc)
{
        struct btrfs_fs_info *fs_info = extent_root->fs_info;
        int ret;
        int slot;
        struct extent_buffer *leaf;
        struct btrfs_key key;

        while (1) {
                ret = btrfs_next_item(extent_root, path);
                if (ret < 0)
                        break;
                if (ret) {
                        ret = 0;
                        break;
                }

                slot = path->slots[0];
                leaf = path->nodes[0];
                btrfs_item_key_to_cpu(leaf, &key, slot);

                if (key.objectid != ctx->bytenr)
                        break;
                if (key.type < BTRFS_TREE_BLOCK_REF_KEY)
                        continue;
                if (key.type > BTRFS_SHARED_DATA_REF_KEY)
                        break;

                switch (key.type) {
                case BTRFS_SHARED_BLOCK_REF_KEY:
                        /* SHARED DIRECT METADATA backref */
                        ret = add_direct_ref(fs_info, preftrees,
                                             info_level + 1, key.offset,
                                             ctx->bytenr, 1, NULL, GFP_NOFS);
                        break;
                case BTRFS_SHARED_DATA_REF_KEY: {
                        /* SHARED DIRECT FULL backref */
                        struct btrfs_shared_data_ref *sdref;
                        int count;

                        sdref = btrfs_item_ptr(leaf, slot,
                                              struct btrfs_shared_data_ref);
                        count = btrfs_shared_data_ref_count(leaf, sdref);
                        ret = add_direct_ref(fs_info, preftrees, 0,
                                             key.offset, ctx->bytenr, count,
                                             sc, GFP_NOFS);
                        break;
                }
                case BTRFS_TREE_BLOCK_REF_KEY:
                        /* NORMAL INDIRECT METADATA backref */
                        ret = add_indirect_ref(fs_info, preftrees, key.offset,
                                               NULL, info_level + 1, ctx->bytenr,
                                               1, NULL, GFP_NOFS);
                        break;
                case BTRFS_EXTENT_DATA_REF_KEY: {
                        /* NORMAL INDIRECT DATA backref */
                        struct btrfs_extent_data_ref *dref;
                        int count;
                        u64 root;

                        dref = btrfs_item_ptr(leaf, slot,
                                              struct btrfs_extent_data_ref);
                        count = btrfs_extent_data_ref_count(leaf, dref);
                        key.objectid = btrfs_extent_data_ref_objectid(leaf,
                                                                      dref);
                        key.type = BTRFS_EXTENT_DATA_KEY;
                        key.offset = btrfs_extent_data_ref_offset(leaf, dref);

                        if (sc && key.objectid != sc->inum &&
                            !sc->have_delayed_delete_refs) {
                                ret = BACKREF_FOUND_SHARED;
                                break;
                        }

                        root = btrfs_extent_data_ref_root(leaf, dref);

                        if (!ctx->skip_data_ref ||
                            !ctx->skip_data_ref(root, key.objectid, key.offset,
                                                ctx->user_ctx))
                                ret = add_indirect_ref(fs_info, preftrees, root,
                                                       &key, 0, ctx->bytenr,
                                                       count, sc, GFP_NOFS);
                        break;
                }
                default:
                        WARN_ON(1);
                }
                if (ret)
                        return ret;

        }

        return ret;
}

/*
 * The caller has joined a transaction or is holding a read lock on the
 * fs_info->commit_root_sem semaphore, so no need to worry about the root's last
 * snapshot field changing while updating or checking the cache.
 */
static bool lookup_backref_shared_cache(struct btrfs_backref_share_check_ctx *ctx,
                                        struct btrfs_root *root,
                                        u64 bytenr, int level, bool *is_shared)
{
        const struct btrfs_fs_info *fs_info = root->fs_info;
        struct btrfs_backref_shared_cache_entry *entry;

        if (!current->journal_info)
                lockdep_assert_held(&fs_info->commit_root_sem);

        if (!ctx->use_path_cache)
                return false;

        if (WARN_ON_ONCE(level >= BTRFS_MAX_LEVEL))
                return false;

        /*
         * Level -1 is used for the data extent, which is not reliable to cache
         * because its reference count can increase or decrease without us
         * realizing. We cache results only for extent buffers that lead from
         * the root node down to the leaf with the file extent item.
         */
        ASSERT(level >= 0);

        entry = &ctx->path_cache_entries[level];

        /* Unused cache entry or being used for some other extent buffer. */
        if (entry->bytenr != bytenr)
                return false;

        /*
         * We cached a false result, but the last snapshot generation of the
         * root changed, so we now have a snapshot. Don't trust the result.
         */
        if (!entry->is_shared &&
            entry->gen != btrfs_root_last_snapshot(&root->root_item))
                return false;

        /*
         * If we cached a true result and the last generation used for dropping
         * a root changed, we can not trust the result, because the dropped root
         * could be a snapshot sharing this extent buffer.
         */
        if (entry->is_shared &&
            entry->gen != btrfs_get_last_root_drop_gen(fs_info))
                return false;

        *is_shared = entry->is_shared;
        /*
         * If the node at this level is shared, than all nodes below are also
         * shared. Currently some of the nodes below may be marked as not shared
         * because we have just switched from one leaf to another, and switched
         * also other nodes above the leaf and below the current level, so mark
         * them as shared.
         */
        if (*is_shared) {
                for (int i = 0; i < level; i++) {
                        ctx->path_cache_entries[i].is_shared = true;
                        ctx->path_cache_entries[i].gen = entry->gen;
                }
        }

        return true;
}

/*
 * The caller has joined a transaction or is holding a read lock on the
 * fs_info->commit_root_sem semaphore, so no need to worry about the root's last
 * snapshot field changing while updating or checking the cache.
 */
static void store_backref_shared_cache(struct btrfs_backref_share_check_ctx *ctx,
                                       struct btrfs_root *root,
                                       u64 bytenr, int level, bool is_shared)
{
        const struct btrfs_fs_info *fs_info = root->fs_info;
        struct btrfs_backref_shared_cache_entry *entry;
        u64 gen;

        if (!current->journal_info)
                lockdep_assert_held(&fs_info->commit_root_sem);

        if (!ctx->use_path_cache)
                return;

        if (WARN_ON_ONCE(level >= BTRFS_MAX_LEVEL))
                return;

        /*
         * Level -1 is used for the data extent, which is not reliable to cache
         * because its reference count can increase or decrease without us
         * realizing. We cache results only for extent buffers that lead from
         * the root node down to the leaf with the file extent item.
         */
        ASSERT(level >= 0);

        if (is_shared)
                gen = btrfs_get_last_root_drop_gen(fs_info);
        else
                gen = btrfs_root_last_snapshot(&root->root_item);

        entry = &ctx->path_cache_entries[level];
        entry->bytenr = bytenr;
        entry->is_shared = is_shared;
        entry->gen = gen;

        /*
         * If we found an extent buffer is shared, set the cache result for all
         * extent buffers below it to true. As nodes in the path are COWed,
         * their sharedness is moved to their children, and if a leaf is COWed,
         * then the sharedness of a data extent becomes direct, the refcount of
         * data extent is increased in the extent item at the extent tree.
         */
        if (is_shared) {
                for (int i = 0; i < level; i++) {
                        entry = &ctx->path_cache_entries[i];
                        entry->is_shared = is_shared;
                        entry->gen = gen;
                }
        }
}

/*
 * this adds all existing backrefs (inline backrefs, backrefs and delayed
 * refs) for the given bytenr to the refs list, merges duplicates and resolves
 * indirect refs to their parent bytenr.
 * When roots are found, they're added to the roots list
 *
 * @ctx:     Backref walking context object, must be not NULL.
 * @sc:      If !NULL, then immediately return BACKREF_FOUND_SHARED when a
 *           shared extent is detected.
 *
 * Otherwise this returns 0 for success and <0 for an error.
 *
 * FIXME some caching might speed things up
 */
static int find_parent_nodes(struct btrfs_backref_walk_ctx *ctx,
                             struct share_check *sc)
{
        struct btrfs_root *root = btrfs_extent_root(ctx->fs_info, ctx->bytenr);
        struct btrfs_key key;
        struct btrfs_path *path;
        struct btrfs_delayed_ref_root *delayed_refs = NULL;
        struct btrfs_delayed_ref_head *head;
        int info_level = 0;
        int ret;
        struct prelim_ref *ref;
        struct rb_node *node;
        struct extent_inode_elem *eie = NULL;
        struct preftrees preftrees = {
                .direct = PREFTREE_INIT,
                .indirect = PREFTREE_INIT,
                .indirect_missing_keys = PREFTREE_INIT
        };

        /* Roots ulist is not needed when using a sharedness check context. */
        if (sc)
                ASSERT(ctx->roots == NULL);

        key.objectid = ctx->bytenr;
        key.offset = (u64)-1;
        if (btrfs_fs_incompat(ctx->fs_info, SKINNY_METADATA))
                key.type = BTRFS_METADATA_ITEM_KEY;
        else
                key.type = BTRFS_EXTENT_ITEM_KEY;

        path = btrfs_alloc_path();
        if (!path)
                return -ENOMEM;
        if (!ctx->trans) {
                path->search_commit_root = 1;
                path->skip_locking = 1;
        }

        if (ctx->time_seq == BTRFS_SEQ_LAST)
                path->skip_locking = 1;

again:
        head = NULL;

        ret = btrfs_search_slot(NULL, root, &key, path, 0, 0);
        if (ret < 0)
                goto out;
        if (ret == 0) {
                /*
                 * Key with offset -1 found, there would have to exist an extent
                 * item with such offset, but this is out of the valid range.
                 */
                ret = -EUCLEAN;
                goto out;
        }

        if (ctx->trans && likely(ctx->trans->type != __TRANS_DUMMY) &&
            ctx->time_seq != BTRFS_SEQ_LAST) {
                /*
                 * We have a specific time_seq we care about and trans which
                 * means we have the path lock, we need to grab the ref head and
                 * lock it so we have a consistent view of the refs at the given
                 * time.
                 */
                delayed_refs = &ctx->trans->transaction->delayed_refs;
                spin_lock(&delayed_refs->lock);
                head = btrfs_find_delayed_ref_head(delayed_refs, ctx->bytenr);
                if (head) {
                        if (!mutex_trylock(&head->mutex)) {
                                refcount_inc(&head->refs);
                                spin_unlock(&delayed_refs->lock);

                                btrfs_release_path(path);

                                /*
                                 * Mutex was contended, block until it's
                                 * released and try again
                                 */
                                mutex_lock(&head->mutex);
                                mutex_unlock(&head->mutex);
                                btrfs_put_delayed_ref_head(head);
                                goto again;
                        }
                        spin_unlock(&delayed_refs->lock);
                        ret = add_delayed_refs(ctx->fs_info, head, ctx->time_seq,
                                               &preftrees, sc);
                        mutex_unlock(&head->mutex);
                        if (ret)
                                goto out;
                } else {
                        spin_unlock(&delayed_refs->lock);
                }
        }

        if (path->slots[0]) {
                struct extent_buffer *leaf;
                int slot;

                path->slots[0]--;
                leaf = path->nodes[0];
                slot = path->slots[0];
                btrfs_item_key_to_cpu(leaf, &key, slot);
                if (key.objectid == ctx->bytenr &&
                    (key.type == BTRFS_EXTENT_ITEM_KEY ||
                     key.type == BTRFS_METADATA_ITEM_KEY)) {
                        ret = add_inline_refs(ctx, path, &info_level,
                                              &preftrees, sc);
                        if (ret)
                                goto out;
                        ret = add_keyed_refs(ctx, root, path, info_level,
                                             &preftrees, sc);
                        if (ret)
                                goto out;
                }
        }

        /*
         * If we have a share context and we reached here, it means the extent
         * is not directly shared (no multiple reference items for it),
         * otherwise we would have exited earlier with a return value of
         * BACKREF_FOUND_SHARED after processing delayed references or while
         * processing inline or keyed references from the extent tree.
         * The extent may however be indirectly shared through shared subtrees
         * as a result from creating snapshots, so we determine below what is
         * its parent node, in case we are dealing with a metadata extent, or
         * what's the leaf (or leaves), from a fs tree, that has a file extent
         * item pointing to it in case we are dealing with a data extent.
         */
        ASSERT(extent_is_shared(sc) == 0);

        /*
         * If we are here for a data extent and we have a share_check structure
         * it means the data extent is not directly shared (does not have
         * multiple reference items), so we have to check if a path in the fs
         * tree (going from the root node down to the leaf that has the file
         * extent item pointing to the data extent) is shared, that is, if any
         * of the extent buffers in the path is referenced by other trees.
         */
        if (sc && ctx->bytenr == sc->data_bytenr) {
                /*
                 * If our data extent is from a generation more recent than the
                 * last generation used to snapshot the root, then we know that
                 * it can not be shared through subtrees, so we can skip
                 * resolving indirect references, there's no point in
                 * determining the extent buffers for the path from the fs tree
                 * root node down to the leaf that has the file extent item that
                 * points to the data extent.
                 */
                if (sc->data_extent_gen >
                    btrfs_root_last_snapshot(&sc->root->root_item)) {
                        ret = BACKREF_FOUND_NOT_SHARED;
                        goto out;
                }

                /*
                 * If we are only determining if a data extent is shared or not
                 * and the corresponding file extent item is located in the same
                 * leaf as the previous file extent item, we can skip resolving
                 * indirect references for a data extent, since the fs tree path
                 * is the same (same leaf, so same path). We skip as long as the
                 * cached result for the leaf is valid and only if there's only
                 * one file extent item pointing to the data extent, because in
                 * the case of multiple file extent items, they may be located
                 * in different leaves and therefore we have multiple paths.
                 */
                if (sc->ctx->curr_leaf_bytenr == sc->ctx->prev_leaf_bytenr &&
                    sc->self_ref_count == 1) {
                        bool cached;
                        bool is_shared;

                        cached = lookup_backref_shared_cache(sc->ctx, sc->root,
                                                     sc->ctx->curr_leaf_bytenr,
                                                     0, &is_shared);
                        if (cached) {
                                if (is_shared)
                                        ret = BACKREF_FOUND_SHARED;
                                else
                                        ret = BACKREF_FOUND_NOT_SHARED;
                                goto out;
                        }
                }
        }

        btrfs_release_path(path);

        ret = add_missing_keys(ctx->fs_info, &preftrees, path->skip_locking == 0);
        if (ret)
                goto out;

        WARN_ON(!RB_EMPTY_ROOT(&preftrees.indirect_missing_keys.root.rb_root));

        ret = resolve_indirect_refs(ctx, path, &preftrees, sc);
        if (ret)
                goto out;

        WARN_ON(!RB_EMPTY_ROOT(&preftrees.indirect.root.rb_root));

        /*
         * This walks the tree of merged and resolved refs. Tree blocks are
         * read in as needed. Unique entries are added to the ulist, and
         * the list of found roots is updated.
         *
         * We release the entire tree in one go before returning.
         */
        node = rb_first_cached(&preftrees.direct.root);
        while (node) {
                ref = rb_entry(node, struct prelim_ref, rbnode);
                node = rb_next(&ref->rbnode);
                /*
                 * ref->count < 0 can happen here if there are delayed
                 * refs with a node->action of BTRFS_DROP_DELAYED_REF.
                 * prelim_ref_insert() relies on this when merging
                 * identical refs to keep the overall count correct.
                 * prelim_ref_insert() will merge only those refs
                 * which compare identically.  Any refs having
                 * e.g. different offsets would not be merged,
                 * and would retain their original ref->count < 0.
                 */
                if (ctx->roots && ref->count && ref->root_id && ref->parent == 0) {
                        /* no parent == root of tree */
                        ret = ulist_add(ctx->roots, ref->root_id, 0, GFP_NOFS);
                        if (ret < 0)
                                goto out;
                }
                if (ref->count && ref->parent) {
                        if (!ctx->skip_inode_ref_list && !ref->inode_list &&
                            ref->level == 0) {
                                struct btrfs_tree_parent_check check = { 0 };
                                struct extent_buffer *eb;

                                check.level = ref->level;

                                eb = read_tree_block(ctx->fs_info, ref->parent,
                                                     &check);
                                if (IS_ERR(eb)) {
                                        ret = PTR_ERR(eb);
                                        goto out;
                                }
                                if (!extent_buffer_uptodate(eb)) {
                                        free_extent_buffer(eb);
                                        ret = -EIO;
                                        goto out;
                                }

                                if (!path->skip_locking)
                                        btrfs_tree_read_lock(eb);
                                ret = find_extent_in_eb(ctx, eb, &eie);
                                if (!path->skip_locking)
                                        btrfs_tree_read_unlock(eb);
                                free_extent_buffer(eb);
                                if (ret == BTRFS_ITERATE_EXTENT_INODES_STOP ||
                                    ret < 0)
                                        goto out;
                                ref->inode_list = eie;
                                /*
                                 * We transferred the list ownership to the ref,
                                 * so set to NULL to avoid a double free in case
                                 * an error happens after this.
                                 */
                                eie = NULL;
                        }
                        ret = ulist_add_merge_ptr(ctx->refs, ref->parent,
                                                  ref->inode_list,
                                                  (void **)&eie, GFP_NOFS);
                        if (ret < 0)
                                goto out;
                        if (!ret && !ctx->skip_inode_ref_list) {
                                /*
                                 * We've recorded that parent, so we must extend
                                 * its inode list here.
                                 *
                                 * However if there was corruption we may not
                                 * have found an eie, return an error in this
                                 * case.
                                 */
                                ASSERT(eie);
                                if (!eie) {
                                        ret = -EUCLEAN;
                                        goto out;
                                }
                                while (eie->next)
                                        eie = eie->next;
                                eie->next = ref->inode_list;
                        }
                        eie = NULL;
                        /*
                         * We have transferred the inode list ownership from
                         * this ref to the ref we added to the 'refs' ulist.
                         * So set this ref's inode list to NULL to avoid
                         * use-after-free when our caller uses it or double
                         * frees in case an error happens before we return.
                         */
                        ref->inode_list = NULL;
                }
                cond_resched();
        }

out:
        btrfs_free_path(path);

        prelim_release(&preftrees.direct);
        prelim_release(&preftrees.indirect);
        prelim_release(&preftrees.indirect_missing_keys);

        if (ret == BTRFS_ITERATE_EXTENT_INODES_STOP || ret < 0)
                free_inode_elem_list(eie);
        return ret;
}

/*
 * Finds all leaves with a reference to the specified combination of
 * @ctx->bytenr and @ctx->extent_item_pos. The bytenr of the found leaves are
 * added to the ulist at @ctx->refs, and that ulist is allocated by this
 * function. The caller should free the ulist with free_leaf_list() if
 * @ctx->ignore_extent_item_pos is false, otherwise a fimple ulist_free() is
 * enough.
 *
 * Returns 0 on success and < 0 on error. On error @ctx->refs is not allocated.
 */
int btrfs_find_all_leafs(struct btrfs_backref_walk_ctx *ctx)
{
        int ret;

        ASSERT(ctx->refs == NULL);

        ctx->refs = ulist_alloc(GFP_NOFS);
        if (!ctx->refs)
                return -ENOMEM;

        ret = find_parent_nodes(ctx, NULL);
        if (ret == BTRFS_ITERATE_EXTENT_INODES_STOP ||
            (ret < 0 && ret != -ENOENT)) {
                free_leaf_list(ctx->refs);
                ctx->refs = NULL;
                return ret;
        }

        return 0;
}

/*
 * Walk all backrefs for a given extent to find all roots that reference this
 * extent. Walking a backref means finding all extents that reference this
 * extent and in turn walk the backrefs of those, too. Naturally this is a
 * recursive process, but here it is implemented in an iterative fashion: We
 * find all referencing extents for the extent in question and put them on a
 * list. In turn, we find all referencing extents for those, further appending
 * to the list. The way we iterate the list allows adding more elements after
 * the current while iterating. The process stops when we reach the end of the
 * list.
 *
 * Found roots are added to @ctx->roots, which is allocated by this function if
 * it points to NULL, in which case the caller is responsible for freeing it
 * after it's not needed anymore.
 * This function requires @ctx->refs to be NULL, as it uses it for allocating a
 * ulist to do temporary work, and frees it before returning.
 *
 * Returns 0 on success, < 0 on error.
 */
static int btrfs_find_all_roots_safe(struct btrfs_backref_walk_ctx *ctx)
{
        const u64 orig_bytenr = ctx->bytenr;
        const bool orig_skip_inode_ref_list = ctx->skip_inode_ref_list;
        bool roots_ulist_allocated = false;
        struct ulist_iterator uiter;
        int ret = 0;

        ASSERT(ctx->refs == NULL);

        ctx->refs = ulist_alloc(GFP_NOFS);
        if (!ctx->refs)
                return -ENOMEM;

        if (!ctx->roots) {
                ctx->roots = ulist_alloc(GFP_NOFS);
                if (!ctx->roots) {
                        ulist_free(ctx->refs);
                        ctx->refs = NULL;
                        return -ENOMEM;
                }
                roots_ulist_allocated = true;
        }

        ctx->skip_inode_ref_list = true;

        ULIST_ITER_INIT(&uiter);
        while (1) {
                struct ulist_node *node;

                ret = find_parent_nodes(ctx, NULL);
                if (ret < 0 && ret != -ENOENT) {
                        if (roots_ulist_allocated) {
                                ulist_free(ctx->roots);
                                ctx->roots = NULL;
                        }
                        break;
                }
                ret = 0;
                node = ulist_next(ctx->refs, &uiter);
                if (!node)
                        break;
                ctx->bytenr = node->val;
                cond_resched();
        }

        ulist_free(ctx->refs);
        ctx->refs = NULL;
        ctx->bytenr = orig_bytenr;
        ctx->skip_inode_ref_list = orig_skip_inode_ref_list;

        return ret;
}

int btrfs_find_all_roots(struct btrfs_backref_walk_ctx *ctx,
                         bool skip_commit_root_sem)
{
        int ret;

        if (!ctx->trans && !skip_commit_root_sem)
                down_read(&ctx->fs_info->commit_root_sem);
        ret = btrfs_find_all_roots_safe(ctx);
        if (!ctx->trans && !skip_commit_root_sem)
                up_read(&ctx->fs_info->commit_root_sem);
        return ret;
}

struct btrfs_backref_share_check_ctx *btrfs_alloc_backref_share_check_ctx(void)
{
        struct btrfs_backref_share_check_ctx *ctx;

        ctx = kzalloc(sizeof(*ctx), GFP_KERNEL);
        if (!ctx)
                return NULL;

        ulist_init(&ctx->refs);

        return ctx;
}

void btrfs_free_backref_share_ctx(struct btrfs_backref_share_check_ctx *ctx)
{
        if (!ctx)
                return;

        ulist_release(&ctx->refs);
        kfree(ctx);
}

/*
 * Check if a data extent is shared or not.
 *
 * @inode:       The inode whose extent we are checking.
 * @bytenr:      Logical bytenr of the extent we are checking.
 * @extent_gen:  Generation of the extent (file extent item) or 0 if it is
 *               not known.
 * @ctx:         A backref sharedness check context.
 *
 * btrfs_is_data_extent_shared uses the backref walking code but will short
 * circuit as soon as it finds a root or inode that doesn't match the
 * one passed in. This provides a significant performance benefit for
 * callers (such as fiemap) which want to know whether the extent is
 * shared but do not need a ref count.
 *
 * This attempts to attach to the running transaction in order to account for
 * delayed refs, but continues on even when no running transaction exists.
 *
 * Return: 0 if extent is not shared, 1 if it is shared, < 0 on error.
 */
int btrfs_is_data_extent_shared(struct btrfs_inode *inode, u64 bytenr,
                                u64 extent_gen,
                                struct btrfs_backref_share_check_ctx *ctx)
{
        struct btrfs_backref_walk_ctx walk_ctx = { 0 };
        struct btrfs_root *root = inode->root;
        struct btrfs_fs_info *fs_info = root->fs_info;
        struct btrfs_trans_handle *trans;
        struct ulist_iterator uiter;
        struct ulist_node *node;
        struct btrfs_seq_list elem = BTRFS_SEQ_LIST_INIT(elem);
        int ret = 0;
        struct share_check shared = {
                .ctx = ctx,
                .root = root,
                .inum = btrfs_ino(inode),
                .data_bytenr = bytenr,
                .data_extent_gen = extent_gen,
                .share_count = 0,
                .self_ref_count = 0,
                .have_delayed_delete_refs = false,
        };
        int level;
        bool leaf_cached;
        bool leaf_is_shared;

        for (int i = 0; i < BTRFS_BACKREF_CTX_PREV_EXTENTS_SIZE; i++) {
                if (ctx->prev_extents_cache[i].bytenr == bytenr)
                        return ctx->prev_extents_cache[i].is_shared;
        }

        ulist_init(&ctx->refs);

        trans = btrfs_join_transaction_nostart(root);
        if (IS_ERR(trans)) {
                if (PTR_ERR(trans) != -ENOENT && PTR_ERR(trans) != -EROFS) {
                        ret = PTR_ERR(trans);
                        goto out;
                }
                trans = NULL;
                down_read(&fs_info->commit_root_sem);
        } else {
                btrfs_get_tree_mod_seq(fs_info, &elem);
                walk_ctx.time_seq = elem.seq;
        }

        ctx->use_path_cache = true;

        /*
         * We may have previously determined that the current leaf is shared.
         * If it is, then we have a data extent that is shared due to a shared
         * subtree (caused by snapshotting) and we don't need to check for data
         * backrefs. If the leaf is not shared, then we must do backref walking
         * to determine if the data extent is shared through reflinks.
         */
        leaf_cached = lookup_backref_shared_cache(ctx, root,
                                                  ctx->curr_leaf_bytenr, 0,
                                                  &leaf_is_shared);
        if (leaf_cached && leaf_is_shared) {
                ret = 1;
                goto out_trans;
        }

        walk_ctx.skip_inode_ref_list = true;
        walk_ctx.trans = trans;
        walk_ctx.fs_info = fs_info;
        walk_ctx.refs = &ctx->refs;

        /* -1 means we are in the bytenr of the data extent. */
        level = -1;
        ULIST_ITER_INIT(&uiter);
        while (1) {
                const unsigned long prev_ref_count = ctx->refs.nnodes;

                walk_ctx.bytenr = bytenr;
                ret = find_parent_nodes(&walk_ctx, &shared);
                if (ret == BACKREF_FOUND_SHARED ||
                    ret == BACKREF_FOUND_NOT_SHARED) {
                        /* If shared must return 1, otherwise return 0. */
                        ret = (ret == BACKREF_FOUND_SHARED) ? 1 : 0;
                        if (level >= 0)
                                store_backref_shared_cache(ctx, root, bytenr,
                                                           level, ret == 1);
                        break;
                }
                if (ret < 0 && ret != -ENOENT)
                        break;
                ret = 0;

                /*
                 * More than one extent buffer (bytenr) may have been added to
                 * the ctx->refs ulist, in which case we have to check multiple
                 * tree paths in case the first one is not shared, so we can not
                 * use the path cache which is made for a single path. Multiple
                 * extent buffers at the current level happen when:
                 *
                 * 1) level -1, the data extent: If our data extent was not
                 *    directly shared (without multiple reference items), then
                 *    it might have a single reference item with a count > 1 for
                 *    the same offset, which means there are 2 (or more) file
                 *    extent items that point to the data extent - this happens
                 *    when a file extent item needs to be split and then one
                 *    item gets moved to another leaf due to a b+tree leaf split
                 *    when inserting some item. In this case the file extent
                 *    items may be located in different leaves and therefore
                 *    some of the leaves may be referenced through shared
                 *    subtrees while others are not. Since our extent buffer
                 *    cache only works for a single path (by far the most common
                 *    case and simpler to deal with), we can not use it if we
                 *    have multiple leaves (which implies multiple paths).
                 *
                 * 2) level >= 0, a tree node/leaf: We can have a mix of direct
                 *    and indirect references on a b+tree node/leaf, so we have
                 *    to check multiple paths, and the extent buffer (the
                 *    current bytenr) may be shared or not. One example is
                 *    during relocation as we may get a shared tree block ref
                 *    (direct ref) and a non-shared tree block ref (indirect
                 *    ref) for the same node/leaf.
                 */
                if ((ctx->refs.nnodes - prev_ref_count) > 1)
                        ctx->use_path_cache = false;

                if (level >= 0)
                        store_backref_shared_cache(ctx, root, bytenr,
                                                   level, false);
                node = ulist_next(&ctx->refs, &uiter);
                if (!node)
                        break;
                bytenr = node->val;
                if (ctx->use_path_cache) {
                        bool is_shared;
                        bool cached;

                        level++;
                        cached = lookup_backref_shared_cache(ctx, root, bytenr,
                                                             level, &is_shared);
                        if (cached) {
                                ret = (is_shared ? 1 : 0);
                                break;
                        }
                }
                shared.share_count = 0;
                shared.have_delayed_delete_refs = false;
                cond_resched();
        }

        /*
         * If the path cache is disabled, then it means at some tree level we
         * got multiple parents due to a mix of direct and indirect backrefs or
         * multiple leaves with file extent items pointing to the same data
         * extent. We have to invalidate the cache and cache only the sharedness
         * result for the levels where we got only one node/reference.
         */
        if (!ctx->use_path_cache) {
                int i = 0;

                level--;
                if (ret >= 0 && level >= 0) {
                        bytenr = ctx->path_cache_entries[level].bytenr;
                        ctx->use_path_cache = true;
                        store_backref_shared_cache(ctx, root, bytenr, level, ret);
                        i = level + 1;
                }

                for ( ; i < BTRFS_MAX_LEVEL; i++)
                        ctx->path_cache_entries[i].bytenr = 0;
        }

        /*
         * Cache the sharedness result for the data extent if we know our inode
         * has more than 1 file extent item that refers to the data extent.
         */
        if (ret >= 0 && shared.self_ref_count > 1) {
                int slot = ctx->prev_extents_cache_slot;

                ctx->prev_extents_cache[slot].bytenr = shared.data_bytenr;
                ctx->prev_extents_cache[slot].is_shared = (ret == 1);

                slot = (slot + 1) % BTRFS_BACKREF_CTX_PREV_EXTENTS_SIZE;
                ctx->prev_extents_cache_slot = slot;
        }

out_trans:
        if (trans) {
                btrfs_put_tree_mod_seq(fs_info, &elem);
                btrfs_end_transaction(trans);
        } else {
                up_read(&fs_info->commit_root_sem);
        }
out:
        ulist_release(&ctx->refs);
        ctx->prev_leaf_bytenr = ctx->curr_leaf_bytenr;

        return ret;
}

int btrfs_find_one_extref(struct btrfs_root *root, u64 inode_objectid,
                          u64 start_off, struct btrfs_path *path,
                          struct btrfs_inode_extref **ret_extref,
                          u64 *found_off)
{
        int ret, slot;
        struct btrfs_key key;
        struct btrfs_key found_key;
        struct btrfs_inode_extref *extref;
        const struct extent_buffer *leaf;
        unsigned long ptr;

        key.objectid = inode_objectid;
        key.type = BTRFS_INODE_EXTREF_KEY;
        key.offset = start_off;

        ret = btrfs_search_slot(NULL, root, &key, path, 0, 0);
        if (ret < 0)
                return ret;

        while (1) {
                leaf = path->nodes[0];
                slot = path->slots[0];
                if (slot >= btrfs_header_nritems(leaf)) {
                        /*
                         * If the item at offset is not found,
                         * btrfs_search_slot will point us to the slot
                         * where it should be inserted. In our case
                         * that will be the slot directly before the
                         * next INODE_REF_KEY_V2 item. In the case
                         * that we're pointing to the last slot in a
                         * leaf, we must move one leaf over.
                         */
                        ret = btrfs_next_leaf(root, path);
                        if (ret) {
                                if (ret >= 1)
                                        ret = -ENOENT;
                                break;
                        }
                        continue;
                }

                btrfs_item_key_to_cpu(leaf, &found_key, slot);

                /*
                 * Check that we're still looking at an extended ref key for
                 * this particular objectid. If we have different
                 * objectid or type then there are no more to be found
                 * in the tree and we can exit.
                 */
                ret = -ENOENT;
                if (found_key.objectid != inode_objectid)
                        break;
                if (found_key.type != BTRFS_INODE_EXTREF_KEY)
                        break;

                ret = 0;
                ptr = btrfs_item_ptr_offset(leaf, path->slots[0]);
                extref = (struct btrfs_inode_extref *)ptr;
                *ret_extref = extref;
                if (found_off)
                        *found_off = found_key.offset;
                break;
        }

        return ret;
}

/*
 * this iterates to turn a name (from iref/extref) into a full filesystem path.
 * Elements of the path are separated by '/' and the path is guaranteed to be
 * 0-terminated. the path is only given within the current file system.
 * Therefore, it never starts with a '/'. the caller is responsible to provide
 * "size" bytes in "dest". the dest buffer will be filled backwards. finally,
 * the start point of the resulting string is returned. this pointer is within
 * dest, normally.
 * in case the path buffer would overflow, the pointer is decremented further
 * as if output was written to the buffer, though no more output is actually
 * generated. that way, the caller can determine how much space would be
 * required for the path to fit into the buffer. in that case, the returned
 * value will be smaller than dest. callers must check this!
 */
char *btrfs_ref_to_path(struct btrfs_root *fs_root, struct btrfs_path *path,
                        u32 name_len, unsigned long name_off,
                        struct extent_buffer *eb_in, u64 parent,
                        char *dest, u32 size)
{
        int slot;
        u64 next_inum;
        int ret;
        s64 bytes_left = ((s64)size) - 1;
        struct extent_buffer *eb = eb_in;
        struct btrfs_key found_key;
        struct btrfs_inode_ref *iref;

        if (bytes_left >= 0)
                dest[bytes_left] = '\0';

        while (1) {
                bytes_left -= name_len;
                if (bytes_left >= 0)
                        read_extent_buffer(eb, dest + bytes_left,
                                           name_off, name_len);
                if (eb != eb_in) {
                        if (!path->skip_locking)
                                btrfs_tree_read_unlock(eb);
                        free_extent_buffer(eb);
                }
                ret = btrfs_find_item(fs_root, path, parent, 0,
                                BTRFS_INODE_REF_KEY, &found_key);
                if (ret > 0)
                        ret = -ENOENT;
                if (ret)
                        break;

                next_inum = found_key.offset;

                /* regular exit ahead */
                if (parent == next_inum)
                        break;

                slot = path->slots[0];
                eb = path->nodes[0];
                /* make sure we can use eb after releasing the path */
                if (eb != eb_in) {
                        path->nodes[0] = NULL;
                        path->locks[0] = 0;
                }
                btrfs_release_path(path);
                iref = btrfs_item_ptr(eb, slot, struct btrfs_inode_ref);

                name_len = btrfs_inode_ref_name_len(eb, iref);
                name_off = (unsigned long)(iref + 1);

                parent = next_inum;
                --bytes_left;
                if (bytes_left >= 0)
                        dest[bytes_left] = '/';
        }

        btrfs_release_path(path);

        if (ret)
                return ERR_PTR(ret);

        return dest + bytes_left;
}

/*
 * this makes the path point to (logical EXTENT_ITEM *)
 * returns BTRFS_EXTENT_FLAG_DATA for data, BTRFS_EXTENT_FLAG_TREE_BLOCK for
 * tree blocks and <0 on error.
 */
int extent_from_logical(struct btrfs_fs_info *fs_info, u64 logical,
                        struct btrfs_path *path, struct btrfs_key *found_key,
                        u64 *flags_ret)
{
        struct btrfs_root *extent_root = btrfs_extent_root(fs_info, logical);
        int ret;
        u64 flags;
        u64 size = 0;
        u32 item_size;
        const struct extent_buffer *eb;
        struct btrfs_extent_item *ei;
        struct btrfs_key key;

        if (btrfs_fs_incompat(fs_info, SKINNY_METADATA))
                key.type = BTRFS_METADATA_ITEM_KEY;
        else
                key.type = BTRFS_EXTENT_ITEM_KEY;
        key.objectid = logical;
        key.offset = (u64)-1;

        ret = btrfs_search_slot(NULL, extent_root, &key, path, 0, 0);
        if (ret < 0)
                return ret;
        if (ret == 0) {
                /*
                 * Key with offset -1 found, there would have to exist an extent
                 * item with such offset, but this is out of the valid range.
                 */
                return -EUCLEAN;
        }

        ret = btrfs_previous_extent_item(extent_root, path, 0);
        if (ret) {
                if (ret > 0)
                        ret = -ENOENT;
                return ret;
        }
        btrfs_item_key_to_cpu(path->nodes[0], found_key, path->slots[0]);
        if (found_key->type == BTRFS_METADATA_ITEM_KEY)
                size = fs_info->nodesize;
        else if (found_key->type == BTRFS_EXTENT_ITEM_KEY)
                size = found_key->offset;

        if (found_key->objectid > logical ||
            found_key->objectid + size <= logical) {
                btrfs_debug(fs_info,
                        "logical %llu is not within any extent", logical);
                return -ENOENT;
        }

        eb = path->nodes[0];
        item_size = btrfs_item_size(eb, path->slots[0]);

        ei = btrfs_item_ptr(eb, path->slots[0], struct btrfs_extent_item);
        flags = btrfs_extent_flags(eb, ei);

        btrfs_debug(fs_info,
                "logical %llu is at position %llu within the extent (%llu EXTENT_ITEM %llu) flags %#llx size %u",
                 logical, logical - found_key->objectid, found_key->objectid,
                 found_key->offset, flags, item_size);

        WARN_ON(!flags_ret);
        if (flags_ret) {
                if (flags & BTRFS_EXTENT_FLAG_TREE_BLOCK)
                        *flags_ret = BTRFS_EXTENT_FLAG_TREE_BLOCK;
                else if (flags & BTRFS_EXTENT_FLAG_DATA)
                        *flags_ret = BTRFS_EXTENT_FLAG_DATA;
                else
                        BUG();
                return 0;
        }

        return -EIO;
}

/*
 * helper function to iterate extent inline refs. ptr must point to a 0 value
 * for the first call and may be modified. it is used to track state.
 * if more refs exist, 0 is returned and the next call to
 * get_extent_inline_ref must pass the modified ptr parameter to get the
 * next ref. after the last ref was processed, 1 is returned.
 * returns <0 on error
 */
static int get_extent_inline_ref(unsigned long *ptr,
                                 const struct extent_buffer *eb,
                                 const struct btrfs_key *key,
                                 const struct btrfs_extent_item *ei,
                                 u32 item_size,
                                 struct btrfs_extent_inline_ref **out_eiref,
                                 int *out_type)
{
        unsigned long end;
        u64 flags;
        struct btrfs_tree_block_info *info;

        if (!*ptr) {
                /* first call */
                flags = btrfs_extent_flags(eb, ei);
                if (flags & BTRFS_EXTENT_FLAG_TREE_BLOCK) {
                        if (key->type == BTRFS_METADATA_ITEM_KEY) {
                                /* a skinny metadata extent */
                                *out_eiref =
                                     (struct btrfs_extent_inline_ref *)(ei + 1);
                        } else {
                                WARN_ON(key->type != BTRFS_EXTENT_ITEM_KEY);
                                info = (struct btrfs_tree_block_info *)(ei + 1);
                                *out_eiref =
                                   (struct btrfs_extent_inline_ref *)(info + 1);
                        }
                } else {
                        *out_eiref = (struct btrfs_extent_inline_ref *)(ei + 1);
                }
                *ptr = (unsigned long)*out_eiref;
                if ((unsigned long)(*ptr) >= (unsigned long)ei + item_size)
                        return -ENOENT;
        }

        end = (unsigned long)ei + item_size;
        *out_eiref = (struct btrfs_extent_inline_ref *)(*ptr);
        *out_type = btrfs_get_extent_inline_ref_type(eb, *out_eiref,
                                                     BTRFS_REF_TYPE_ANY);
        if (*out_type == BTRFS_REF_TYPE_INVALID)
                return -EUCLEAN;

        *ptr += btrfs_extent_inline_ref_size(*out_type);
        WARN_ON(*ptr > end);
        if (*ptr == end)
                return 1; /* last */

        return 0;
}

/*
 * reads the tree block backref for an extent. tree level and root are returned
 * through out_level and out_root. ptr must point to a 0 value for the first
 * call and may be modified (see get_extent_inline_ref comment).
 * returns 0 if data was provided, 1 if there was no more data to provide or
 * <0 on error.
 */
int tree_backref_for_extent(unsigned long *ptr, struct extent_buffer *eb,
                            struct btrfs_key *key, struct btrfs_extent_item *ei,
                            u32 item_size, u64 *out_root, u8 *out_level)
{
        int ret;
        int type;
        struct btrfs_extent_inline_ref *eiref;

        if (*ptr == (unsigned long)-1)
                return 1;

        while (1) {
                ret = get_extent_inline_ref(ptr, eb, key, ei, item_size,
                                              &eiref, &type);
                if (ret < 0)
                        return ret;

                if (type == BTRFS_TREE_BLOCK_REF_KEY ||
                    type == BTRFS_SHARED_BLOCK_REF_KEY)
                        break;

                if (ret == 1)
                        return 1;
        }

        /* we can treat both ref types equally here */
        *out_root = btrfs_extent_inline_ref_offset(eb, eiref);

        if (key->type == BTRFS_EXTENT_ITEM_KEY) {
                struct btrfs_tree_block_info *info;

                info = (struct btrfs_tree_block_info *)(ei + 1);
                *out_level = btrfs_tree_block_level(eb, info);
        } else {
                ASSERT(key->type == BTRFS_METADATA_ITEM_KEY);
                *out_level = (u8)key->offset;
        }

        if (ret == 1)
                *ptr = (unsigned long)-1;

        return 0;
}

static int iterate_leaf_refs(struct btrfs_fs_info *fs_info,
                             struct extent_inode_elem *inode_list,
                             u64 root, u64 extent_item_objectid,
                             iterate_extent_inodes_t *iterate, void *ctx)
{
        struct extent_inode_elem *eie;
        int ret = 0;

        for (eie = inode_list; eie; eie = eie->next) {
                btrfs_debug(fs_info,
                            "ref for %llu resolved, key (%llu EXTEND_DATA %llu), root %llu",
                            extent_item_objectid, eie->inum,
                            eie->offset, root);
                ret = iterate(eie->inum, eie->offset, eie->num_bytes, root, ctx);
                if (ret) {
                        btrfs_debug(fs_info,
                                    "stopping iteration for %llu due to ret=%d",
                                    extent_item_objectid, ret);
                        break;
                }
        }

        return ret;
}

/*
 * calls iterate() for every inode that references the extent identified by
 * the given parameters.
 * when the iterator function returns a non-zero value, iteration stops.
 */
int iterate_extent_inodes(struct btrfs_backref_walk_ctx *ctx,
                          bool search_commit_root,
                          iterate_extent_inodes_t *iterate, void *user_ctx)
{
        int ret;
        struct ulist *refs;
        struct ulist_node *ref_node;
        struct btrfs_seq_list seq_elem = BTRFS_SEQ_LIST_INIT(seq_elem);
        struct ulist_iterator ref_uiter;

        btrfs_debug(ctx->fs_info, "resolving all inodes for extent %llu",
                    ctx->bytenr);

        ASSERT(ctx->trans == NULL);
        ASSERT(ctx->roots == NULL);

        if (!search_commit_root) {
                struct btrfs_trans_handle *trans;

                trans = btrfs_attach_transaction(ctx->fs_info->tree_root);
                if (IS_ERR(trans)) {
                        if (PTR_ERR(trans) != -ENOENT &&
                            PTR_ERR(trans) != -EROFS)
                                return PTR_ERR(trans);
                        trans = NULL;
                }
                ctx->trans = trans;
        }

        if (ctx->trans) {
                btrfs_get_tree_mod_seq(ctx->fs_info, &seq_elem);
                ctx->time_seq = seq_elem.seq;
        } else {
                down_read(&ctx->fs_info->commit_root_sem);
        }

        ret = btrfs_find_all_leafs(ctx);
        if (ret)
                goto out;
        refs = ctx->refs;
        ctx->refs = NULL;

        ULIST_ITER_INIT(&ref_uiter);
        while (!ret && (ref_node = ulist_next(refs, &ref_uiter))) {
                const u64 leaf_bytenr = ref_node->val;
                struct ulist_node *root_node;
                struct ulist_iterator root_uiter;
                struct extent_inode_elem *inode_list;

                inode_list = (struct extent_inode_elem *)(uintptr_t)ref_node->aux;

                if (ctx->cache_lookup) {
                        const u64 *root_ids;
                        int root_count;
                        bool cached;

                        cached = ctx->cache_lookup(leaf_bytenr, ctx->user_ctx,
                                                   &root_ids, &root_count);
                        if (cached) {
                                for (int i = 0; i < root_count; i++) {
                                        ret = iterate_leaf_refs(ctx->fs_info,
                                                                inode_list,
                                                                root_ids[i],
                                                                leaf_bytenr,
                                                                iterate,
                                                                user_ctx);
                                        if (ret)
                                                break;
                                }
                                continue;
                        }
                }

                if (!ctx->roots) {
                        ctx->roots = ulist_alloc(GFP_NOFS);
                        if (!ctx->roots) {
                                ret = -ENOMEM;
                                break;
                        }
                }

                ctx->bytenr = leaf_bytenr;
                ret = btrfs_find_all_roots_safe(ctx);
                if (ret)
                        break;

                if (ctx->cache_store)
                        ctx->cache_store(leaf_bytenr, ctx->roots, ctx->user_ctx);

                ULIST_ITER_INIT(&root_uiter);
                while (!ret && (root_node = ulist_next(ctx->roots, &root_uiter))) {
                        btrfs_debug(ctx->fs_info,
                                    "root %llu references leaf %llu, data list %#llx",
                                    root_node->val, ref_node->val,
                                    ref_node->aux);
                        ret = iterate_leaf_refs(ctx->fs_info, inode_list,
                                                root_node->val, ctx->bytenr,
                                                iterate, user_ctx);
                }
                ulist_reinit(ctx->roots);
        }

        free_leaf_list(refs);
out:
        if (ctx->trans) {
                btrfs_put_tree_mod_seq(ctx->fs_info, &seq_elem);
                btrfs_end_transaction(ctx->trans);
                ctx->trans = NULL;
        } else {
                up_read(&ctx->fs_info->commit_root_sem);
        }

        ulist_free(ctx->roots);
        ctx->roots = NULL;

        if (ret == BTRFS_ITERATE_EXTENT_INODES_STOP)
                ret = 0;

        return ret;
}

static int build_ino_list(u64 inum, u64 offset, u64 num_bytes, u64 root, void *ctx)
{
        struct btrfs_data_container *inodes = ctx;
        const size_t c = 3 * sizeof(u64);

        if (inodes->bytes_left >= c) {
                inodes->bytes_left -= c;
                inodes->val[inodes->elem_cnt] = inum;
                inodes->val[inodes->elem_cnt + 1] = offset;
                inodes->val[inodes->elem_cnt + 2] = root;
                inodes->elem_cnt += 3;
        } else {
                inodes->bytes_missing += c - inodes->bytes_left;
                inodes->bytes_left = 0;
                inodes->elem_missed += 3;
        }

        return 0;
}

int iterate_inodes_from_logical(u64 logical, struct btrfs_fs_info *fs_info,
                                struct btrfs_path *path,
                                void *ctx, bool ignore_offset)
{
        struct btrfs_backref_walk_ctx walk_ctx = { 0 };
        int ret;
        u64 flags = 0;
        struct btrfs_key found_key;
        int search_commit_root = path->search_commit_root;

        ret = extent_from_logical(fs_info, logical, path, &found_key, &flags);
        btrfs_release_path(path);
        if (ret < 0)
                return ret;
        if (flags & BTRFS_EXTENT_FLAG_TREE_BLOCK)
                return -EINVAL;

        walk_ctx.bytenr = found_key.objectid;
        if (ignore_offset)
                walk_ctx.ignore_extent_item_pos = true;
        else
                walk_ctx.extent_item_pos = logical - found_key.objectid;
        walk_ctx.fs_info = fs_info;

        return iterate_extent_inodes(&walk_ctx, search_commit_root,
                                     build_ino_list, ctx);
}

static int inode_to_path(u64 inum, u32 name_len, unsigned long name_off,
                         struct extent_buffer *eb, struct inode_fs_paths *ipath);

static int iterate_inode_refs(u64 inum, struct inode_fs_paths *ipath)
{
        int ret = 0;
        int slot;
        u32 cur;
        u32 len;
        u32 name_len;
        u64 parent = 0;
        int found = 0;
        struct btrfs_root *fs_root = ipath->fs_root;
        struct btrfs_path *path = ipath->btrfs_path;
        struct extent_buffer *eb;
        struct btrfs_inode_ref *iref;
        struct btrfs_key found_key;

        while (!ret) {
                ret = btrfs_find_item(fs_root, path, inum,
                                parent ? parent + 1 : 0, BTRFS_INODE_REF_KEY,
                                &found_key);

                if (ret < 0)
                        break;
                if (ret) {
                        ret = found ? 0 : -ENOENT;
                        break;
                }
                ++found;

                parent = found_key.offset;
                slot = path->slots[0];
                eb = btrfs_clone_extent_buffer(path->nodes[0]);
                if (!eb) {
                        ret = -ENOMEM;
                        break;
                }
                btrfs_release_path(path);

                iref = btrfs_item_ptr(eb, slot, struct btrfs_inode_ref);

                for (cur = 0; cur < btrfs_item_size(eb, slot); cur += len) {
                        name_len = btrfs_inode_ref_name_len(eb, iref);
                        /* path must be released before calling iterate()! */
                        btrfs_debug(fs_root->fs_info,
                                "following ref at offset %u for inode %llu in tree %llu",
                                cur, found_key.objectid,
                                fs_root->root_key.objectid);
                        ret = inode_to_path(parent, name_len,
                                      (unsigned long)(iref + 1), eb, ipath);
                        if (ret)
                                break;
                        len = sizeof(*iref) + name_len;
                        iref = (struct btrfs_inode_ref *)((char *)iref + len);
                }
                free_extent_buffer(eb);
        }

        btrfs_release_path(path);

        return ret;
}

static int iterate_inode_extrefs(u64 inum, struct inode_fs_paths *ipath)
{
        int ret;
        int slot;
        u64 offset = 0;
        u64 parent;
        int found = 0;
        struct btrfs_root *fs_root = ipath->fs_root;
        struct btrfs_path *path = ipath->btrfs_path;
        struct extent_buffer *eb;
        struct btrfs_inode_extref *extref;
        u32 item_size;
        u32 cur_offset;
        unsigned long ptr;

        while (1) {
                ret = btrfs_find_one_extref(fs_root, inum, offset, path, &extref,
                                            &offset);
                if (ret < 0)
                        break;
                if (ret) {
                        ret = found ? 0 : -ENOENT;
                        break;
                }
                ++found;

                slot = path->slots[0];
                eb = btrfs_clone_extent_buffer(path->nodes[0]);
                if (!eb) {
                        ret = -ENOMEM;
                        break;
                }
                btrfs_release_path(path);

                item_size = btrfs_item_size(eb, slot);
                ptr = btrfs_item_ptr_offset(eb, slot);
                cur_offset = 0;

                while (cur_offset < item_size) {
                        u32 name_len;

                        extref = (struct btrfs_inode_extref *)(ptr + cur_offset);
                        parent = btrfs_inode_extref_parent(eb, extref);
                        name_len = btrfs_inode_extref_name_len(eb, extref);
                        ret = inode_to_path(parent, name_len,
                                      (unsigned long)&extref->name, eb, ipath);
                        if (ret)
                                break;

                        cur_offset += btrfs_inode_extref_name_len(eb, extref);
                        cur_offset += sizeof(*extref);
                }
                free_extent_buffer(eb);

                offset++;
        }

        btrfs_release_path(path);

        return ret;
}

/*
 * returns 0 if the path could be dumped (probably truncated)
 * returns <0 in case of an error
 */
static int inode_to_path(u64 inum, u32 name_len, unsigned long name_off,
                         struct extent_buffer *eb, struct inode_fs_paths *ipath)
{
        char *fspath;
        char *fspath_min;
        int i = ipath->fspath->elem_cnt;
        const int s_ptr = sizeof(char *);
        u32 bytes_left;

        bytes_left = ipath->fspath->bytes_left > s_ptr ?
                                        ipath->fspath->bytes_left - s_ptr : 0;

        fspath_min = (char *)ipath->fspath->val + (i + 1) * s_ptr;
        fspath = btrfs_ref_to_path(ipath->fs_root, ipath->btrfs_path, name_len,
                                   name_off, eb, inum, fspath_min, bytes_left);
        if (IS_ERR(fspath))
                return PTR_ERR(fspath);

        if (fspath > fspath_min) {
                ipath->fspath->val[i] = (u64)(unsigned long)fspath;
                ++ipath->fspath->elem_cnt;
                ipath->fspath->bytes_left = fspath - fspath_min;
        } else {
                ++ipath->fspath->elem_missed;
                ipath->fspath->bytes_missing += fspath_min - fspath;
                ipath->fspath->bytes_left = 0;
        }

        return 0;
}

/*
 * this dumps all file system paths to the inode into the ipath struct, provided
 * is has been created large enough. each path is zero-terminated and accessed
 * from ipath->fspath->val[i].
 * when it returns, there are ipath->fspath->elem_cnt number of paths available
 * in ipath->fspath->val[]. when the allocated space wasn't sufficient, the
 * number of missed paths is recorded in ipath->fspath->elem_missed, otherwise,
 * it's zero. ipath->fspath->bytes_missing holds the number of bytes that would
 * have been needed to return all paths.
 */
int paths_from_inode(u64 inum, struct inode_fs_paths *ipath)
{
        int ret;
        int found_refs = 0;

        ret = iterate_inode_refs(inum, ipath);
        if (!ret)
                ++found_refs;
        else if (ret != -ENOENT)
                return ret;

        ret = iterate_inode_extrefs(inum, ipath);
        if (ret == -ENOENT && found_refs)
                return 0;

        return ret;
}

struct btrfs_data_container *init_data_container(u32 total_bytes)
{
        struct btrfs_data_container *data;
        size_t alloc_bytes;

        alloc_bytes = max_t(size_t, total_bytes, sizeof(*data));
        data = kvmalloc(alloc_bytes, GFP_KERNEL);
        if (!data)
                return ERR_PTR(-ENOMEM);

        if (total_bytes >= sizeof(*data)) {
                data->bytes_left = total_bytes - sizeof(*data);
                data->bytes_missing = 0;
        } else {
                data->bytes_missing = sizeof(*data) - total_bytes;
                data->bytes_left = 0;
        }

        data->elem_cnt = 0;
        data->elem_missed = 0;

        return data;
}

/*
 * allocates space to return multiple file system paths for an inode.
 * total_bytes to allocate are passed, note that space usable for actual path
 * information will be total_bytes - sizeof(struct inode_fs_paths).
 * the returned pointer must be freed with free_ipath() in the end.
 */
struct inode_fs_paths *init_ipath(s32 total_bytes, struct btrfs_root *fs_root,
                                        struct btrfs_path *path)
{
        struct inode_fs_paths *ifp;
        struct btrfs_data_container *fspath;

        fspath = init_data_container(total_bytes);
        if (IS_ERR(fspath))
                return ERR_CAST(fspath);

        ifp = kmalloc(sizeof(*ifp), GFP_KERNEL);
        if (!ifp) {
                kvfree(fspath);
                return ERR_PTR(-ENOMEM);
        }

        ifp->btrfs_path = path;
        ifp->fspath = fspath;
        ifp->fs_root = fs_root;

        return ifp;
}

void free_ipath(struct inode_fs_paths *ipath)
{
        if (!ipath)
                return;
        kvfree(ipath->fspath);
        kfree(ipath);
}

struct btrfs_backref_iter *btrfs_backref_iter_alloc(struct btrfs_fs_info *fs_info)
{
        struct btrfs_backref_iter *ret;

        ret = kzalloc(sizeof(*ret), GFP_NOFS);
        if (!ret)
                return NULL;

        ret->path = btrfs_alloc_path();
        if (!ret->path) {
                kfree(ret);
                return NULL;
        }

        /* Current backref iterator only supports iteration in commit root */
        ret->path->search_commit_root = 1;
        ret->path->skip_locking = 1;
        ret->fs_info = fs_info;

        return ret;
}

static void btrfs_backref_iter_release(struct btrfs_backref_iter *iter)
{
        iter->bytenr = 0;
        iter->item_ptr = 0;
        iter->cur_ptr = 0;
        iter->end_ptr = 0;
        btrfs_release_path(iter->path);
        memset(&iter->cur_key, 0, sizeof(iter->cur_key));
}

int btrfs_backref_iter_start(struct btrfs_backref_iter *iter, u64 bytenr)
{
        struct btrfs_fs_info *fs_info = iter->fs_info;
        struct btrfs_root *extent_root = btrfs_extent_root(fs_info, bytenr);
        struct btrfs_path *path = iter->path;
        struct btrfs_extent_item *ei;
        struct btrfs_key key;
        int ret;

        key.objectid = bytenr;
        key.type = BTRFS_METADATA_ITEM_KEY;
        key.offset = (u64)-1;
        iter->bytenr = bytenr;

        ret = btrfs_search_slot(NULL, extent_root, &key, path, 0, 0);
        if (ret < 0)
                return ret;
        if (ret == 0) {
                /*
                 * Key with offset -1 found, there would have to exist an extent
                 * item with such offset, but this is out of the valid range.
                 */
                ret = -EUCLEAN;
                goto release;
        }
        if (path->slots[0] == 0) {
                WARN_ON(IS_ENABLED(CONFIG_BTRFS_DEBUG));
                ret = -EUCLEAN;
                goto release;
        }
        path->slots[0]--;

        btrfs_item_key_to_cpu(path->nodes[0], &key, path->slots[0]);
        if ((key.type != BTRFS_EXTENT_ITEM_KEY &&
             key.type != BTRFS_METADATA_ITEM_KEY) || key.objectid != bytenr) {
                ret = -ENOENT;
                goto release;
        }
        memcpy(&iter->cur_key, &key, sizeof(key));
        iter->item_ptr = (u32)btrfs_item_ptr_offset(path->nodes[0],
                                                    path->slots[0]);
        iter->end_ptr = (u32)(iter->item_ptr +
                        btrfs_item_size(path->nodes[0], path->slots[0]));
        ei = btrfs_item_ptr(path->nodes[0], path->slots[0],
                            struct btrfs_extent_item);

        /*
         * Only support iteration on tree backref yet.
         *
         * This is an extra precaution for non skinny-metadata, where
         * EXTENT_ITEM is also used for tree blocks, that we can only use
         * extent flags to determine if it's a tree block.
         */
        if (btrfs_extent_flags(path->nodes[0], ei) & BTRFS_EXTENT_FLAG_DATA) {
                ret = -ENOTSUPP;
                goto release;
        }
        iter->cur_ptr = (u32)(iter->item_ptr + sizeof(*ei));

        /* If there is no inline backref, go search for keyed backref */
        if (iter->cur_ptr >= iter->end_ptr) {
                ret = btrfs_next_item(extent_root, path);

                /* No inline nor keyed ref */
                if (ret > 0) {
                        ret = -ENOENT;
                        goto release;
                }
                if (ret < 0)
                        goto release;

                btrfs_item_key_to_cpu(path->nodes[0], &iter->cur_key,
                                path->slots[0]);
                if (iter->cur_key.objectid != bytenr ||
                    (iter->cur_key.type != BTRFS_SHARED_BLOCK_REF_KEY &&
                     iter->cur_key.type != BTRFS_TREE_BLOCK_REF_KEY)) {
                        ret = -ENOENT;
                        goto release;
                }
                iter->cur_ptr = (u32)btrfs_item_ptr_offset(path->nodes[0],
                                                           path->slots[0]);
                iter->item_ptr = iter->cur_ptr;
                iter->end_ptr = (u32)(iter->item_ptr + btrfs_item_size(
                                      path->nodes[0], path->slots[0]));
        }

        return 0;
release:
        btrfs_backref_iter_release(iter);
        return ret;
}

static bool btrfs_backref_iter_is_inline_ref(struct btrfs_backref_iter *iter)
{
        if (iter->cur_key.type == BTRFS_EXTENT_ITEM_KEY ||
            iter->cur_key.type == BTRFS_METADATA_ITEM_KEY)
                return true;
        return false;
}

/*
 * Go to the next backref item of current bytenr, can be either inlined or
 * keyed.
 *
 * Caller needs to check whether it's inline ref or not by iter->cur_key.
 *
 * Return 0 if we get next backref without problem.
 * Return >0 if there is no extra backref for this bytenr.
 * Return <0 if there is something wrong happened.
 */
int btrfs_backref_iter_next(struct btrfs_backref_iter *iter)
{
        struct extent_buffer *eb = iter->path->nodes[0];
        struct btrfs_root *extent_root;
        struct btrfs_path *path = iter->path;
        struct btrfs_extent_inline_ref *iref;
        int ret;
        u32 size;

        if (btrfs_backref_iter_is_inline_ref(iter)) {
                /* We're still inside the inline refs */
                ASSERT(iter->cur_ptr < iter->end_ptr);

                if (btrfs_backref_has_tree_block_info(iter)) {
                        /* First tree block info */
                        size = sizeof(struct btrfs_tree_block_info);
                } else {
                        /* Use inline ref type to determine the size */
                        int type;

                        iref = (struct btrfs_extent_inline_ref *)
                                ((unsigned long)iter->cur_ptr);
                        type = btrfs_extent_inline_ref_type(eb, iref);

                        size = btrfs_extent_inline_ref_size(type);
                }
                iter->cur_ptr += size;
                if (iter->cur_ptr < iter->end_ptr)
                        return 0;

                /* All inline items iterated, fall through */
        }

        /* We're at keyed items, there is no inline item, go to the next one */
        extent_root = btrfs_extent_root(iter->fs_info, iter->bytenr);
        ret = btrfs_next_item(extent_root, iter->path);
        if (ret)
                return ret;

        btrfs_item_key_to_cpu(path->nodes[0], &iter->cur_key, path->slots[0]);
        if (iter->cur_key.objectid != iter->bytenr ||
            (iter->cur_key.type != BTRFS_TREE_BLOCK_REF_KEY &&
             iter->cur_key.type != BTRFS_SHARED_BLOCK_REF_KEY))
                return 1;
        iter->item_ptr = (u32)btrfs_item_ptr_offset(path->nodes[0],
                                        path->slots[0]);
        iter->cur_ptr = iter->item_ptr;
        iter->end_ptr = iter->item_ptr + (u32)btrfs_item_size(path->nodes[0],
                                                path->slots[0]);
        return 0;
}

void btrfs_backref_init_cache(struct btrfs_fs_info *fs_info,
                              struct btrfs_backref_cache *cache, bool is_reloc)
{
        int i;

        cache->rb_root = RB_ROOT;
        for (i = 0; i < BTRFS_MAX_LEVEL; i++)
                INIT_LIST_HEAD(&cache->pending[i]);
        INIT_LIST_HEAD(&cache->changed);
        INIT_LIST_HEAD(&cache->detached);
        INIT_LIST_HEAD(&cache->leaves);
        INIT_LIST_HEAD(&cache->pending_edge);
        INIT_LIST_HEAD(&cache->useless_node);
        cache->fs_info = fs_info;
        cache->is_reloc = is_reloc;
}

struct btrfs_backref_node *btrfs_backref_alloc_node(
                struct btrfs_backref_cache *cache, u64 bytenr, int level)
{
        struct btrfs_backref_node *node;

        ASSERT(level >= 0 && level < BTRFS_MAX_LEVEL);
        node = kzalloc(sizeof(*node), GFP_NOFS);
        if (!node)
                return node;

        INIT_LIST_HEAD(&node->list);
        INIT_LIST_HEAD(&node->upper);
        INIT_LIST_HEAD(&node->lower);
        RB_CLEAR_NODE(&node->rb_node);
        cache->nr_nodes++;
        node->level = level;
        node->bytenr = bytenr;

        return node;
}

void btrfs_backref_free_node(struct btrfs_backref_cache *cache,
                             struct btrfs_backref_node *node)
{
        if (node) {
                ASSERT(list_empty(&node->list));
                ASSERT(list_empty(&node->lower));
                ASSERT(node->eb == NULL);
                cache->nr_nodes--;
                btrfs_put_root(node->root);
                kfree(node);
        }
}

struct btrfs_backref_edge *btrfs_backref_alloc_edge(
                struct btrfs_backref_cache *cache)
{
        struct btrfs_backref_edge *edge;

        edge = kzalloc(sizeof(*edge), GFP_NOFS);
        if (edge)
                cache->nr_edges++;
        return edge;
}

void btrfs_backref_free_edge(struct btrfs_backref_cache *cache,
                             struct btrfs_backref_edge *edge)
{
        if (edge) {
                cache->nr_edges--;
                kfree(edge);
        }
}

void btrfs_backref_unlock_node_buffer(struct btrfs_backref_node *node)
{
        if (node->locked) {
                btrfs_tree_unlock(node->eb);
                node->locked = 0;
        }
}

void btrfs_backref_drop_node_buffer(struct btrfs_backref_node *node)
{
        if (node->eb) {
                btrfs_backref_unlock_node_buffer(node);
                free_extent_buffer(node->eb);
                node->eb = NULL;
        }
}

/*
 * Drop the backref node from cache without cleaning up its children
 * edges.
 *
 * This can only be called on node without parent edges.
 * The children edges are still kept as is.
 */
void btrfs_backref_drop_node(struct btrfs_backref_cache *tree,
                             struct btrfs_backref_node *node)
{
        ASSERT(list_empty(&node->upper));

        btrfs_backref_drop_node_buffer(node);
        list_del_init(&node->list);
        list_del_init(&node->lower);
        if (!RB_EMPTY_NODE(&node->rb_node))
                rb_erase(&node->rb_node, &tree->rb_root);
        btrfs_backref_free_node(tree, node);
}

/*
 * Drop the backref node from cache, also cleaning up all its
 * upper edges and any uncached nodes in the path.
 *
 * This cleanup happens bottom up, thus the node should either
 * be the lowest node in the cache or a detached node.
 */
void btrfs_backref_cleanup_node(struct btrfs_backref_cache *cache,
                                struct btrfs_backref_node *node)
{
        struct btrfs_backref_node *upper;
        struct btrfs_backref_edge *edge;

        if (!node)
                return;

        BUG_ON(!node->lowest && !node->detached);
        while (!list_empty(&node->upper)) {
                edge = list_entry(node->upper.next, struct btrfs_backref_edge,
                                  list[LOWER]);
                upper = edge->node[UPPER];
                list_del(&edge->list[LOWER]);
                list_del(&edge->list[UPPER]);
                btrfs_backref_free_edge(cache, edge);

                /*
                 * Add the node to leaf node list if no other child block
                 * cached.
                 */
                if (list_empty(&upper->lower)) {
                        list_add_tail(&upper->lower, &cache->leaves);
                        upper->lowest = 1;
                }
        }

        btrfs_backref_drop_node(cache, node);
}

/*
 * Release all nodes/edges from current cache
 */
void btrfs_backref_release_cache(struct btrfs_backref_cache *cache)
{
        struct btrfs_backref_node *node;
        int i;

        while (!list_empty(&cache->detached)) {
                node = list_entry(cache->detached.next,
                                  struct btrfs_backref_node, list);
                btrfs_backref_cleanup_node(cache, node);
        }

        while (!list_empty(&cache->leaves)) {
                node = list_entry(cache->leaves.next,
                                  struct btrfs_backref_node, lower);
                btrfs_backref_cleanup_node(cache, node);
        }

        cache->last_trans = 0;

        for (i = 0; i < BTRFS_MAX_LEVEL; i++)
                ASSERT(list_empty(&cache->pending[i]));
        ASSERT(list_empty(&cache->pending_edge));
        ASSERT(list_empty(&cache->useless_node));
        ASSERT(list_empty(&cache->changed));
        ASSERT(list_empty(&cache->detached));
        ASSERT(RB_EMPTY_ROOT(&cache->rb_root));
        ASSERT(!cache->nr_nodes);
        ASSERT(!cache->nr_edges);
}

void btrfs_backref_link_edge(struct btrfs_backref_edge *edge,
                             struct btrfs_backref_node *lower,
                             struct btrfs_backref_node *upper,
                             int link_which)
{
        ASSERT(upper && lower && upper->level == lower->level + 1);
        edge->node[LOWER] = lower;
        edge->node[UPPER] = upper;
        if (link_which & LINK_LOWER)
                list_add_tail(&edge->list[LOWER], &lower->upper);
        if (link_which & LINK_UPPER)
                list_add_tail(&edge->list[UPPER], &upper->lower);
}
/*
 * Handle direct tree backref
 *
 * Direct tree backref means, the backref item shows its parent bytenr
 * directly. This is for SHARED_BLOCK_REF backref (keyed or inlined).
 *
 * @ref_key:    The converted backref key.
 *              For keyed backref, it's the item key.
 *              For inlined backref, objectid is the bytenr,
 *              type is btrfs_inline_ref_type, offset is
 *              btrfs_inline_ref_offset.
 */
static int handle_direct_tree_backref(struct btrfs_backref_cache *cache,
                                      struct btrfs_key *ref_key,
                                      struct btrfs_backref_node *cur)
{
        struct btrfs_backref_edge *edge;
        struct btrfs_backref_node *upper;
        struct rb_node *rb_node;

        ASSERT(ref_key->type == BTRFS_SHARED_BLOCK_REF_KEY);

        /* Only reloc root uses backref pointing to itself */
        if (ref_key->objectid == ref_key->offset) {
                struct btrfs_root *root;

                cur->is_reloc_root = 1;
                /* Only reloc backref cache cares about a specific root */
                if (cache->is_reloc) {
                        root = find_reloc_root(cache->fs_info, cur->bytenr);
                        if (!root)
                                return -ENOENT;
                        cur->root = root;
                } else {
                        /*
                         * For generic purpose backref cache, reloc root node
                         * is useless.
                         */
                        list_add(&cur->list, &cache->useless_node);
                }
                return 0;
        }

        edge = btrfs_backref_alloc_edge(cache);
        if (!edge)
                return -ENOMEM;

        rb_node = rb_simple_search(&cache->rb_root, ref_key->offset);
        if (!rb_node) {
                /* Parent node not yet cached */
                upper = btrfs_backref_alloc_node(cache, ref_key->offset,
                                           cur->level + 1);
                if (!upper) {
                        btrfs_backref_free_edge(cache, edge);
                        return -ENOMEM;
                }

                /*
                 *  Backrefs for the upper level block isn't cached, add the
                 *  block to pending list
                 */
                list_add_tail(&edge->list[UPPER], &cache->pending_edge);
        } else {
                /* Parent node already cached */
                upper = rb_entry(rb_node, struct btrfs_backref_node, rb_node);
                ASSERT(upper->checked);
                INIT_LIST_HEAD(&edge->list[UPPER]);
        }
        btrfs_backref_link_edge(edge, cur, upper, LINK_LOWER);
        return 0;
}

/*
 * Handle indirect tree backref
 *
 * Indirect tree backref means, we only know which tree the node belongs to.
 * We still need to do a tree search to find out the parents. This is for
 * TREE_BLOCK_REF backref (keyed or inlined).
 *
 * @trans:      Transaction handle.
 * @ref_key:    The same as @ref_key in  handle_direct_tree_backref()
 * @tree_key:   The first key of this tree block.
 * @path:       A clean (released) path, to avoid allocating path every time
 *              the function get called.
 */
static int handle_indirect_tree_backref(struct btrfs_trans_handle *trans,
                                        struct btrfs_backref_cache *cache,
                                        struct btrfs_path *path,
                                        struct btrfs_key *ref_key,
                                        struct btrfs_key *tree_key,
                                        struct btrfs_backref_node *cur)
{
        struct btrfs_fs_info *fs_info = cache->fs_info;
        struct btrfs_backref_node *upper;
        struct btrfs_backref_node *lower;
        struct btrfs_backref_edge *edge;
        struct extent_buffer *eb;
        struct btrfs_root *root;
        struct rb_node *rb_node;
        int level;
        bool need_check = true;
        int ret;

        root = btrfs_get_fs_root(fs_info, ref_key->offset, false);
        if (IS_ERR(root))
                return PTR_ERR(root);
        if (!test_bit(BTRFS_ROOT_SHAREABLE, &root->state))
                cur->cowonly = 1;

        if (btrfs_root_level(&root->root_item) == cur->level) {
                /* Tree root */
                ASSERT(btrfs_root_bytenr(&root->root_item) == cur->bytenr);
                /*
                 * For reloc backref cache, we may ignore reloc root.  But for
                 * general purpose backref cache, we can't rely on
                 * btrfs_should_ignore_reloc_root() as it may conflict with
                 * current running relocation and lead to missing root.
                 *
                 * For general purpose backref cache, reloc root detection is
                 * completely relying on direct backref (key->offset is parent
                 * bytenr), thus only do such check for reloc cache.
                 */
                if (btrfs_should_ignore_reloc_root(root) && cache->is_reloc) {
                        btrfs_put_root(root);
                        list_add(&cur->list, &cache->useless_node);
                } else {
                        cur->root = root;
                }
                return 0;
        }

        level = cur->level + 1;

        /* Search the tree to find parent blocks referring to the block */
        path->search_commit_root = 1;
        path->skip_locking = 1;
        path->lowest_level = level;
        ret = btrfs_search_slot(NULL, root, tree_key, path, 0, 0);
        path->lowest_level = 0;
        if (ret < 0) {
                btrfs_put_root(root);
                return ret;
        }
        if (ret > 0 && path->slots[level] > 0)
                path->slots[level]--;

        eb = path->nodes[level];
        if (btrfs_node_blockptr(eb, path->slots[level]) != cur->bytenr) {
                btrfs_err(fs_info,
"couldn't find block (%llu) (level %d) in tree (%llu) with key (%llu %u %llu)",
                          cur->bytenr, level - 1, root->root_key.objectid,
                          tree_key->objectid, tree_key->type, tree_key->offset);
                btrfs_put_root(root);
                ret = -ENOENT;
                goto out;
        }
        lower = cur;

        /* Add all nodes and edges in the path */
        for (; level < BTRFS_MAX_LEVEL; level++) {
                if (!path->nodes[level]) {
                        ASSERT(btrfs_root_bytenr(&root->root_item) ==
                               lower->bytenr);
                        /* Same as previous should_ignore_reloc_root() call */
                        if (btrfs_should_ignore_reloc_root(root) &&
                            cache->is_reloc) {
                                btrfs_put_root(root);
                                list_add(&lower->list, &cache->useless_node);
                        } else {
                                lower->root = root;
                        }
                        break;
                }

                edge = btrfs_backref_alloc_edge(cache);
                if (!edge) {
                        btrfs_put_root(root);
                        ret = -ENOMEM;
                        goto out;
                }

                eb = path->nodes[level];
                rb_node = rb_simple_search(&cache->rb_root, eb->start);
                if (!rb_node) {
                        upper = btrfs_backref_alloc_node(cache, eb->start,
                                                         lower->level + 1);
                        if (!upper) {
                                btrfs_put_root(root);
                                btrfs_backref_free_edge(cache, edge);
                                ret = -ENOMEM;
                                goto out;
                        }
                        upper->owner = btrfs_header_owner(eb);
                        if (!test_bit(BTRFS_ROOT_SHAREABLE, &root->state))
                                upper->cowonly = 1;

                        /*
                         * If we know the block isn't shared we can avoid
                         * checking its backrefs.
                         */
                        if (btrfs_block_can_be_shared(trans, root, eb))
                                upper->checked = 0;
                        else
                                upper->checked = 1;

                        /*
                         * Add the block to pending list if we need to check its
                         * backrefs, we only do this once while walking up a
                         * tree as we will catch anything else later on.
                         */
                        if (!upper->checked && need_check) {
                                need_check = false;
                                list_add_tail(&edge->list[UPPER],
                                              &cache->pending_edge);
                        } else {
                                if (upper->checked)
                                        need_check = true;
                                INIT_LIST_HEAD(&edge->list[UPPER]);
                        }
                } else {
                        upper = rb_entry(rb_node, struct btrfs_backref_node,
                                         rb_node);
                        ASSERT(upper->checked);
                        INIT_LIST_HEAD(&edge->list[UPPER]);
                        if (!upper->owner)
                                upper->owner = btrfs_header_owner(eb);
                }
                btrfs_backref_link_edge(edge, lower, upper, LINK_LOWER);

                if (rb_node) {
                        btrfs_put_root(root);
                        break;
                }
                lower = upper;
                upper = NULL;
        }
out:
        btrfs_release_path(path);
        return ret;
}

/*
 * Add backref node @cur into @cache.
 *
 * NOTE: Even if the function returned 0, @cur is not yet cached as its upper
 *       links aren't yet bi-directional. Needs to finish such links.
 *       Use btrfs_backref_finish_upper_links() to finish such linkage.
 *
 * @trans:      Transaction handle.
 * @path:       Released path for indirect tree backref lookup
 * @iter:       Released backref iter for extent tree search
 * @node_key:   The first key of the tree block
 */
int btrfs_backref_add_tree_node(struct btrfs_trans_handle *trans,
                                struct btrfs_backref_cache *cache,
                                struct btrfs_path *path,
                                struct btrfs_backref_iter *iter,
                                struct btrfs_key *node_key,
                                struct btrfs_backref_node *cur)
{
        struct btrfs_backref_edge *edge;
        struct btrfs_backref_node *exist;
        int ret;

        ret = btrfs_backref_iter_start(iter, cur->bytenr);
        if (ret < 0)
                return ret;
        /*
         * We skip the first btrfs_tree_block_info, as we don't use the key
         * stored in it, but fetch it from the tree block
         */
        if (btrfs_backref_has_tree_block_info(iter)) {
                ret = btrfs_backref_iter_next(iter);
                if (ret < 0)
                        goto out;
                /* No extra backref? This means the tree block is corrupted */
                if (ret > 0) {
                        ret = -EUCLEAN;
                        goto out;
                }
        }
        WARN_ON(cur->checked);
        if (!list_empty(&cur->upper)) {
                /*
                 * The backref was added previously when processing backref of
                 * type BTRFS_TREE_BLOCK_REF_KEY
                 */
                ASSERT(list_is_singular(&cur->upper));
                edge = list_entry(cur->upper.next, struct btrfs_backref_edge,
                                  list[LOWER]);
                ASSERT(list_empty(&edge->list[UPPER]));
                exist = edge->node[UPPER];
                /*
                 * Add the upper level block to pending list if we need check
                 * its backrefs
                 */
                if (!exist->checked)
                        list_add_tail(&edge->list[UPPER], &cache->pending_edge);
        } else {
                exist = NULL;
        }

        for (; ret == 0; ret = btrfs_backref_iter_next(iter)) {
                struct extent_buffer *eb;
                struct btrfs_key key;
                int type;

                cond_resched();
                eb = iter->path->nodes[0];

                key.objectid = iter->bytenr;
                if (btrfs_backref_iter_is_inline_ref(iter)) {
                        struct btrfs_extent_inline_ref *iref;

                        /* Update key for inline backref */
                        iref = (struct btrfs_extent_inline_ref *)
                                ((unsigned long)iter->cur_ptr);
                        type = btrfs_get_extent_inline_ref_type(eb, iref,
                                                        BTRFS_REF_TYPE_BLOCK);
                        if (type == BTRFS_REF_TYPE_INVALID) {
                                ret = -EUCLEAN;
                                goto out;
                        }
                        key.type = type;
                        key.offset = btrfs_extent_inline_ref_offset(eb, iref);
                } else {
                        key.type = iter->cur_key.type;
                        key.offset = iter->cur_key.offset;
                }

                /*
                 * Parent node found and matches current inline ref, no need to
                 * rebuild this node for this inline ref
                 */
                if (exist &&
                    ((key.type == BTRFS_TREE_BLOCK_REF_KEY &&
                      exist->owner == key.offset) ||
                     (key.type == BTRFS_SHARED_BLOCK_REF_KEY &&
                      exist->bytenr == key.offset))) {
                        exist = NULL;
                        continue;
                }

                /* SHARED_BLOCK_REF means key.offset is the parent bytenr */
                if (key.type == BTRFS_SHARED_BLOCK_REF_KEY) {
                        ret = handle_direct_tree_backref(cache, &key, cur);
                        if (ret < 0)
                                goto out;
                } else if (key.type == BTRFS_TREE_BLOCK_REF_KEY) {
                        /*
                         * key.type == BTRFS_TREE_BLOCK_REF_KEY, inline ref
                         * offset means the root objectid. We need to search
                         * the tree to get its parent bytenr.
                         */
                        ret = handle_indirect_tree_backref(trans, cache, path,
                                                           &key, node_key, cur);
                        if (ret < 0)
                                goto out;
                }
                /*
                 * Unrecognized tree backref items (if it can pass tree-checker)
                 * would be ignored.
                 */
        }
        ret = 0;
        cur->checked = 1;
        WARN_ON(exist);
out:
        btrfs_backref_iter_release(iter);
        return ret;
}

/*
 * Finish the upwards linkage created by btrfs_backref_add_tree_node()
 */
int btrfs_backref_finish_upper_links(struct btrfs_backref_cache *cache,
                                     struct btrfs_backref_node *start)
{
        struct list_head *useless_node = &cache->useless_node;
        struct btrfs_backref_edge *edge;
        struct rb_node *rb_node;
        LIST_HEAD(pending_edge);

        ASSERT(start->checked);

        /* Insert this node to cache if it's not COW-only */
        if (!start->cowonly) {
                rb_node = rb_simple_insert(&cache->rb_root, start->bytenr,
                                           &start->rb_node);
                if (rb_node)
                        btrfs_backref_panic(cache->fs_info, start->bytenr,
                                            -EEXIST);
                list_add_tail(&start->lower, &cache->leaves);
        }

        /*
         * Use breadth first search to iterate all related edges.
         *
         * The starting points are all the edges of this node
         */
        list_for_each_entry(edge, &start->upper, list[LOWER])
                list_add_tail(&edge->list[UPPER], &pending_edge);

        while (!list_empty(&pending_edge)) {
                struct btrfs_backref_node *upper;
                struct btrfs_backref_node *lower;

                edge = list_first_entry(&pending_edge,
                                struct btrfs_backref_edge, list[UPPER]);
                list_del_init(&edge->list[UPPER]);
                upper = edge->node[UPPER];
                lower = edge->node[LOWER];

                /* Parent is detached, no need to keep any edges */
                if (upper->detached) {
                        list_del(&edge->list[LOWER]);
                        btrfs_backref_free_edge(cache, edge);

                        /* Lower node is orphan, queue for cleanup */
                        if (list_empty(&lower->upper))
                                list_add(&lower->list, useless_node);
                        continue;
                }

                /*
                 * All new nodes added in current build_backref_tree() haven't
                 * been linked to the cache rb tree.
                 * So if we have upper->rb_node populated, this means a cache
                 * hit. We only need to link the edge, as @upper and all its
                 * parents have already been linked.
                 */
                if (!RB_EMPTY_NODE(&upper->rb_node)) {
                        if (upper->lowest) {
                                list_del_init(&upper->lower);
                                upper->lowest = 0;
                        }

                        list_add_tail(&edge->list[UPPER], &upper->lower);
                        continue;
                }

                /* Sanity check, we shouldn't have any unchecked nodes */
                if (!upper->checked) {
                        ASSERT(0);
                        return -EUCLEAN;
                }

                /* Sanity check, COW-only node has non-COW-only parent */
                if (start->cowonly != upper->cowonly) {
                        ASSERT(0);
                        return -EUCLEAN;
                }

                /* Only cache non-COW-only (subvolume trees) tree blocks */
                if (!upper->cowonly) {
                        rb_node = rb_simple_insert(&cache->rb_root, upper->bytenr,
                                                   &upper->rb_node);
                        if (rb_node) {
                                btrfs_backref_panic(cache->fs_info,
                                                upper->bytenr, -EEXIST);
                                return -EUCLEAN;
                        }
                }

                list_add_tail(&edge->list[UPPER], &upper->lower);

                /*
                 * Also queue all the parent edges of this uncached node
                 * to finish the upper linkage
                 */
                list_for_each_entry(edge, &upper->upper, list[LOWER])
                        list_add_tail(&edge->list[UPPER], &pending_edge);
        }
        return 0;
}

void btrfs_backref_error_cleanup(struct btrfs_backref_cache *cache,
                                 struct btrfs_backref_node *node)
{
        struct btrfs_backref_node *lower;
        struct btrfs_backref_node *upper;
        struct btrfs_backref_edge *edge;

        while (!list_empty(&cache->useless_node)) {
                lower = list_first_entry(&cache->useless_node,
                                   struct btrfs_backref_node, list);
                list_del_init(&lower->list);
        }
        while (!list_empty(&cache->pending_edge)) {
                edge = list_first_entry(&cache->pending_edge,
                                struct btrfs_backref_edge, list[UPPER]);
                list_del(&edge->list[UPPER]);
                list_del(&edge->list[LOWER]);
                lower = edge->node[LOWER];
                upper = edge->node[UPPER];
                btrfs_backref_free_edge(cache, edge);

                /*
                 * Lower is no longer linked to any upper backref nodes and
                 * isn't in the cache, we can free it ourselves.
                 */
                if (list_empty(&lower->upper) &&
                    RB_EMPTY_NODE(&lower->rb_node))
                        list_add(&lower->list, &cache->useless_node);

                if (!RB_EMPTY_NODE(&upper->rb_node))
                        continue;

                /* Add this guy's upper edges to the list to process */
                list_for_each_entry(edge, &upper->upper, list[LOWER])
                        list_add_tail(&edge->list[UPPER],
                                      &cache->pending_edge);
                if (list_empty(&upper->upper))
                        list_add(&upper->list, &cache->useless_node);
        }

        while (!list_empty(&cache->useless_node)) {
                lower = list_first_entry(&cache->useless_node,
                                   struct btrfs_backref_node, list);
                list_del_init(&lower->list);
                if (lower == node)
                        node = NULL;
                btrfs_backref_drop_node(cache, lower);
        }

        btrfs_backref_cleanup_node(cache, node);
        ASSERT(list_empty(&cache->useless_node) &&
               list_empty(&cache->pending_edge));
}