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

// SPDX-License-Identifier: GPL-2.0

#include <linux/err.h>
#include <linux/slab.h>
#include <linux/spinlock.h>
#include "messages.h"
#include "ctree.h"
#include "volumes.h"
#include "extent_map.h"
#include "compression.h"
#include "btrfs_inode.h"


static struct kmem_cache *extent_map_cache;

int __init extent_map_init(void)
{
        extent_map_cache = kmem_cache_create("btrfs_extent_map",
                        sizeof(struct extent_map), 0,
                        SLAB_MEM_SPREAD, NULL);
        if (!extent_map_cache)
                return -ENOMEM;
        return 0;
}

void __cold extent_map_exit(void)
{
        kmem_cache_destroy(extent_map_cache);
}

/*
 * Initialize the extent tree @tree.  Should be called for each new inode or
 * other user of the extent_map interface.
 */
void extent_map_tree_init(struct extent_map_tree *tree)
{
        tree->map = RB_ROOT_CACHED;
        INIT_LIST_HEAD(&tree->modified_extents);
        rwlock_init(&tree->lock);
}

/*
 * Allocate a new extent_map structure.  The new structure is returned with a
 * reference count of one and needs to be freed using free_extent_map()
 */
struct extent_map *alloc_extent_map(void)
{
        struct extent_map *em;
        em = kmem_cache_zalloc(extent_map_cache, GFP_NOFS);
        if (!em)
                return NULL;
        RB_CLEAR_NODE(&em->rb_node);
        em->compress_type = BTRFS_COMPRESS_NONE;
        refcount_set(&em->refs, 1);
        INIT_LIST_HEAD(&em->list);
        return em;
}

/*
 * Drop the reference out on @em by one and free the structure if the reference
 * count hits zero.
 */
void free_extent_map(struct extent_map *em)
{
        if (!em)
                return;
        if (refcount_dec_and_test(&em->refs)) {
                WARN_ON(extent_map_in_tree(em));
                WARN_ON(!list_empty(&em->list));
                if (test_bit(EXTENT_FLAG_FS_MAPPING, &em->flags))
                        kfree(em->map_lookup);
                kmem_cache_free(extent_map_cache, em);
        }
}

/* Do the math around the end of an extent, handling wrapping. */
static u64 range_end(u64 start, u64 len)
{
        if (start + len < start)
                return (u64)-1;
        return start + len;
}

static int tree_insert(struct rb_root_cached *root, struct extent_map *em)
{
        struct rb_node **p = &root->rb_root.rb_node;
        struct rb_node *parent = NULL;
        struct extent_map *entry = NULL;
        struct rb_node *orig_parent = NULL;
        u64 end = range_end(em->start, em->len);
        bool leftmost = true;

        while (*p) {
                parent = *p;
                entry = rb_entry(parent, struct extent_map, rb_node);

                if (em->start < entry->start) {
                        p = &(*p)->rb_left;
                } else if (em->start >= extent_map_end(entry)) {
                        p = &(*p)->rb_right;
                        leftmost = false;
                } else {
                        return -EEXIST;
                }
        }

        orig_parent = parent;
        while (parent && em->start >= extent_map_end(entry)) {
                parent = rb_next(parent);
                entry = rb_entry(parent, struct extent_map, rb_node);
        }
        if (parent)
                if (end > entry->start && em->start < extent_map_end(entry))
                        return -EEXIST;

        parent = orig_parent;
        entry = rb_entry(parent, struct extent_map, rb_node);
        while (parent && em->start < entry->start) {
                parent = rb_prev(parent);
                entry = rb_entry(parent, struct extent_map, rb_node);
        }
        if (parent)
                if (end > entry->start && em->start < extent_map_end(entry))
                        return -EEXIST;

        rb_link_node(&em->rb_node, orig_parent, p);
        rb_insert_color_cached(&em->rb_node, root, leftmost);
        return 0;
}

/*
 * Search through the tree for an extent_map with a given offset.  If it can't
 * be found, try to find some neighboring extents
 */
static struct rb_node *__tree_search(struct rb_root *root, u64 offset,
                                     struct rb_node **prev_or_next_ret)
{
        struct rb_node *n = root->rb_node;
        struct rb_node *prev = NULL;
        struct rb_node *orig_prev = NULL;
        struct extent_map *entry;
        struct extent_map *prev_entry = NULL;

        ASSERT(prev_or_next_ret);

        while (n) {
                entry = rb_entry(n, struct extent_map, rb_node);
                prev = n;
                prev_entry = entry;

                if (offset < entry->start)
                        n = n->rb_left;
                else if (offset >= extent_map_end(entry))
                        n = n->rb_right;
                else
                        return n;
        }

        orig_prev = prev;
        while (prev && offset >= extent_map_end(prev_entry)) {
                prev = rb_next(prev);
                prev_entry = rb_entry(prev, struct extent_map, rb_node);
        }

        /*
         * Previous extent map found, return as in this case the caller does not
         * care about the next one.
         */
        if (prev) {
                *prev_or_next_ret = prev;
                return NULL;
        }

        prev = orig_prev;
        prev_entry = rb_entry(prev, struct extent_map, rb_node);
        while (prev && offset < prev_entry->start) {
                prev = rb_prev(prev);
                prev_entry = rb_entry(prev, struct extent_map, rb_node);
        }
        *prev_or_next_ret = prev;

        return NULL;
}

static inline u64 extent_map_block_end(const struct extent_map *em)
{
        if (em->block_start + em->block_len < em->block_start)
                return (u64)-1;
        return em->block_start + em->block_len;
}

/* Check to see if two extent_map structs are adjacent and safe to merge. */
static int mergable_maps(struct extent_map *prev, struct extent_map *next)
{
        if (test_bit(EXTENT_FLAG_PINNED, &prev->flags))
                return 0;

        /*
         * don't merge compressed extents, we need to know their
         * actual size
         */
        if (test_bit(EXTENT_FLAG_COMPRESSED, &prev->flags))
                return 0;

        if (test_bit(EXTENT_FLAG_LOGGING, &prev->flags) ||
            test_bit(EXTENT_FLAG_LOGGING, &next->flags))
                return 0;

        /*
         * We don't want to merge stuff that hasn't been written to the log yet
         * since it may not reflect exactly what is on disk, and that would be
         * bad.
         */
        if (!list_empty(&prev->list) || !list_empty(&next->list))
                return 0;

        ASSERT(next->block_start != EXTENT_MAP_DELALLOC &&
               prev->block_start != EXTENT_MAP_DELALLOC);

        if (prev->map_lookup || next->map_lookup)
                ASSERT(test_bit(EXTENT_FLAG_FS_MAPPING, &prev->flags) &&
                       test_bit(EXTENT_FLAG_FS_MAPPING, &next->flags));

        if (extent_map_end(prev) == next->start &&
            prev->flags == next->flags &&
            prev->map_lookup == next->map_lookup &&
            ((next->block_start == EXTENT_MAP_HOLE &&
              prev->block_start == EXTENT_MAP_HOLE) ||
             (next->block_start == EXTENT_MAP_INLINE &&
              prev->block_start == EXTENT_MAP_INLINE) ||
             (next->block_start < EXTENT_MAP_LAST_BYTE - 1 &&
              next->block_start == extent_map_block_end(prev)))) {
                return 1;
        }
        return 0;
}

static void try_merge_map(struct extent_map_tree *tree, struct extent_map *em)
{
        struct extent_map *merge = NULL;
        struct rb_node *rb;

        /*
         * We can't modify an extent map that is in the tree and that is being
         * used by another task, as it can cause that other task to see it in
         * inconsistent state during the merging. We always have 1 reference for
         * the tree and 1 for this task (which is unpinning the extent map or
         * clearing the logging flag), so anything > 2 means it's being used by
         * other tasks too.
         */
        if (refcount_read(&em->refs) > 2)
                return;

        if (em->start != 0) {
                rb = rb_prev(&em->rb_node);
                if (rb)
                        merge = rb_entry(rb, struct extent_map, rb_node);
                if (rb && mergable_maps(merge, em)) {
                        em->start = merge->start;
                        em->orig_start = merge->orig_start;
                        em->len += merge->len;
                        em->block_len += merge->block_len;
                        em->block_start = merge->block_start;
                        em->mod_len = (em->mod_len + em->mod_start) - merge->mod_start;
                        em->mod_start = merge->mod_start;
                        em->generation = max(em->generation, merge->generation);
                        set_bit(EXTENT_FLAG_MERGED, &em->flags);

                        rb_erase_cached(&merge->rb_node, &tree->map);
                        RB_CLEAR_NODE(&merge->rb_node);
                        free_extent_map(merge);
                }
        }

        rb = rb_next(&em->rb_node);
        if (rb)
                merge = rb_entry(rb, struct extent_map, rb_node);
        if (rb && mergable_maps(em, merge)) {
                em->len += merge->len;
                em->block_len += merge->block_len;
                rb_erase_cached(&merge->rb_node, &tree->map);
                RB_CLEAR_NODE(&merge->rb_node);
                em->mod_len = (merge->mod_start + merge->mod_len) - em->mod_start;
                em->generation = max(em->generation, merge->generation);
                set_bit(EXTENT_FLAG_MERGED, &em->flags);
                free_extent_map(merge);
        }
}

/*
 * Unpin an extent from the cache.
 *
 * @tree:       tree to unpin the extent in
 * @start:      logical offset in the file
 * @len:        length of the extent
 * @gen:        generation that this extent has been modified in
 *
 * Called after an extent has been written to disk properly.  Set the generation
 * to the generation that actually added the file item to the inode so we know
 * we need to sync this extent when we call fsync().
 */
int unpin_extent_cache(struct extent_map_tree *tree, u64 start, u64 len,
                       u64 gen)
{
        int ret = 0;
        struct extent_map *em;
        bool prealloc = false;

        write_lock(&tree->lock);
        em = lookup_extent_mapping(tree, start, len);

        WARN_ON(!em || em->start != start);

        if (!em)
                goto out;

        em->generation = gen;
        clear_bit(EXTENT_FLAG_PINNED, &em->flags);
        em->mod_start = em->start;
        em->mod_len = em->len;

        if (test_bit(EXTENT_FLAG_FILLING, &em->flags)) {
                prealloc = true;
                clear_bit(EXTENT_FLAG_FILLING, &em->flags);
        }

        try_merge_map(tree, em);

        if (prealloc) {
                em->mod_start = em->start;
                em->mod_len = em->len;
        }

        free_extent_map(em);
out:
        write_unlock(&tree->lock);
        return ret;

}

void clear_em_logging(struct extent_map_tree *tree, struct extent_map *em)
{
        lockdep_assert_held_write(&tree->lock);

        clear_bit(EXTENT_FLAG_LOGGING, &em->flags);
        if (extent_map_in_tree(em))
                try_merge_map(tree, em);
}

static inline void setup_extent_mapping(struct extent_map_tree *tree,
                                        struct extent_map *em,
                                        int modified)
{
        refcount_inc(&em->refs);
        em->mod_start = em->start;
        em->mod_len = em->len;

        if (modified)
                list_move(&em->list, &tree->modified_extents);
        else
                try_merge_map(tree, em);
}

static void extent_map_device_set_bits(struct extent_map *em, unsigned bits)
{
        struct map_lookup *map = em->map_lookup;
        u64 stripe_size = em->orig_block_len;
        int i;

        for (i = 0; i < map->num_stripes; i++) {
                struct btrfs_io_stripe *stripe = &map->stripes[i];
                struct btrfs_device *device = stripe->dev;

                set_extent_bit(&device->alloc_state, stripe->physical,
                               stripe->physical + stripe_size - 1,
                               bits | EXTENT_NOWAIT, NULL);
        }
}

static void extent_map_device_clear_bits(struct extent_map *em, unsigned bits)
{
        struct map_lookup *map = em->map_lookup;
        u64 stripe_size = em->orig_block_len;
        int i;

        for (i = 0; i < map->num_stripes; i++) {
                struct btrfs_io_stripe *stripe = &map->stripes[i];
                struct btrfs_device *device = stripe->dev;

                __clear_extent_bit(&device->alloc_state, stripe->physical,
                                   stripe->physical + stripe_size - 1,
                                   bits | EXTENT_NOWAIT,
                                   NULL, NULL);
        }
}

/*
 * Add new extent map to the extent tree
 *
 * @tree:       tree to insert new map in
 * @em:         map to insert
 * @modified:   indicate whether the given @em should be added to the
 *              modified list, which indicates the extent needs to be logged
 *
 * Insert @em into @tree or perform a simple forward/backward merge with
 * existing mappings.  The extent_map struct passed in will be inserted
 * into the tree directly, with an additional reference taken, or a
 * reference dropped if the merge attempt was successful.
 */
int add_extent_mapping(struct extent_map_tree *tree,
                       struct extent_map *em, int modified)
{
        int ret = 0;

        lockdep_assert_held_write(&tree->lock);

        ret = tree_insert(&tree->map, em);
        if (ret)
                goto out;

        setup_extent_mapping(tree, em, modified);
        if (test_bit(EXTENT_FLAG_FS_MAPPING, &em->flags)) {
                extent_map_device_set_bits(em, CHUNK_ALLOCATED);
                extent_map_device_clear_bits(em, CHUNK_TRIMMED);
        }
out:
        return ret;
}

static struct extent_map *
__lookup_extent_mapping(struct extent_map_tree *tree,
                        u64 start, u64 len, int strict)
{
        struct extent_map *em;
        struct rb_node *rb_node;
        struct rb_node *prev_or_next = NULL;
        u64 end = range_end(start, len);

        rb_node = __tree_search(&tree->map.rb_root, start, &prev_or_next);
        if (!rb_node) {
                if (prev_or_next)
                        rb_node = prev_or_next;
                else
                        return NULL;
        }

        em = rb_entry(rb_node, struct extent_map, rb_node);

        if (strict && !(end > em->start && start < extent_map_end(em)))
                return NULL;

        refcount_inc(&em->refs);
        return em;
}

/*
 * Lookup extent_map that intersects @start + @len range.
 *
 * @tree:       tree to lookup in
 * @start:      byte offset to start the search
 * @len:        length of the lookup range
 *
 * Find and return the first extent_map struct in @tree that intersects the
 * [start, len] range.  There may be additional objects in the tree that
 * intersect, so check the object returned carefully to make sure that no
 * additional lookups are needed.
 */
struct extent_map *lookup_extent_mapping(struct extent_map_tree *tree,
                                         u64 start, u64 len)
{
        return __lookup_extent_mapping(tree, start, len, 1);
}

/*
 * Find a nearby extent map intersecting @start + @len (not an exact search).
 *
 * @tree:       tree to lookup in
 * @start:      byte offset to start the search
 * @len:        length of the lookup range
 *
 * Find and return the first extent_map struct in @tree that intersects the
 * [start, len] range.
 *
 * If one can't be found, any nearby extent may be returned
 */
struct extent_map *search_extent_mapping(struct extent_map_tree *tree,
                                         u64 start, u64 len)
{
        return __lookup_extent_mapping(tree, start, len, 0);
}

/*
 * Remove an extent_map from the extent tree.
 *
 * @tree:       extent tree to remove from
 * @em:         extent map being removed
 *
 * Remove @em from @tree.  No reference counts are dropped, and no checks
 * are done to see if the range is in use.
 */
void remove_extent_mapping(struct extent_map_tree *tree, struct extent_map *em)
{
        lockdep_assert_held_write(&tree->lock);

        WARN_ON(test_bit(EXTENT_FLAG_PINNED, &em->flags));
        rb_erase_cached(&em->rb_node, &tree->map);
        if (!test_bit(EXTENT_FLAG_LOGGING, &em->flags))
                list_del_init(&em->list);
        if (test_bit(EXTENT_FLAG_FS_MAPPING, &em->flags))
                extent_map_device_clear_bits(em, CHUNK_ALLOCATED);
        RB_CLEAR_NODE(&em->rb_node);
}

static void replace_extent_mapping(struct extent_map_tree *tree,
                                   struct extent_map *cur,
                                   struct extent_map *new,
                                   int modified)
{
        lockdep_assert_held_write(&tree->lock);

        WARN_ON(test_bit(EXTENT_FLAG_PINNED, &cur->flags));
        ASSERT(extent_map_in_tree(cur));
        if (!test_bit(EXTENT_FLAG_LOGGING, &cur->flags))
                list_del_init(&cur->list);
        rb_replace_node_cached(&cur->rb_node, &new->rb_node, &tree->map);
        RB_CLEAR_NODE(&cur->rb_node);

        setup_extent_mapping(tree, new, modified);
}

static struct extent_map *next_extent_map(const struct extent_map *em)
{
        struct rb_node *next;

        next = rb_next(&em->rb_node);
        if (!next)
                return NULL;
        return container_of(next, struct extent_map, rb_node);
}

static struct extent_map *prev_extent_map(struct extent_map *em)
{
        struct rb_node *prev;

        prev = rb_prev(&em->rb_node);
        if (!prev)
                return NULL;
        return container_of(prev, struct extent_map, rb_node);
}

/*
 * Helper for btrfs_get_extent.  Given an existing extent in the tree,
 * the existing extent is the nearest extent to map_start,
 * and an extent that you want to insert, deal with overlap and insert
 * the best fitted new extent into the tree.
 */
static noinline int merge_extent_mapping(struct extent_map_tree *em_tree,
                                         struct extent_map *existing,
                                         struct extent_map *em,
                                         u64 map_start)
{
        struct extent_map *prev;
        struct extent_map *next;
        u64 start;
        u64 end;
        u64 start_diff;

        BUG_ON(map_start < em->start || map_start >= extent_map_end(em));

        if (existing->start > map_start) {
                next = existing;
                prev = prev_extent_map(next);
        } else {
                prev = existing;
                next = next_extent_map(prev);
        }

        start = prev ? extent_map_end(prev) : em->start;
        start = max_t(u64, start, em->start);
        end = next ? next->start : extent_map_end(em);
        end = min_t(u64, end, extent_map_end(em));
        start_diff = start - em->start;
        em->start = start;
        em->len = end - start;
        if (em->block_start < EXTENT_MAP_LAST_BYTE &&
            !test_bit(EXTENT_FLAG_COMPRESSED, &em->flags)) {
                em->block_start += start_diff;
                em->block_len = em->len;
        }
        return add_extent_mapping(em_tree, em, 0);
}

/*
 * Add extent mapping into em_tree.
 *
 * @fs_info:  the filesystem
 * @em_tree:  extent tree into which we want to insert the extent mapping
 * @em_in:    extent we are inserting
 * @start:    start of the logical range btrfs_get_extent() is requesting
 * @len:      length of the logical range btrfs_get_extent() is requesting
 *
 * Note that @em_in's range may be different from [start, start+len),
 * but they must be overlapped.
 *
 * Insert @em_in into @em_tree. In case there is an overlapping range, handle
 * the -EEXIST by either:
 * a) Returning the existing extent in @em_in if @start is within the
 *    existing em.
 * b) Merge the existing extent with @em_in passed in.
 *
 * Return 0 on success, otherwise -EEXIST.
 *
 */
int btrfs_add_extent_mapping(struct btrfs_fs_info *fs_info,
                             struct extent_map_tree *em_tree,
                             struct extent_map **em_in, u64 start, u64 len)
{
        int ret;
        struct extent_map *em = *em_in;

        /*
         * Tree-checker should have rejected any inline extent with non-zero
         * file offset. Here just do a sanity check.
         */
        if (em->block_start == EXTENT_MAP_INLINE)
                ASSERT(em->start == 0);

        ret = add_extent_mapping(em_tree, em, 0);
        /* it is possible that someone inserted the extent into the tree
         * while we had the lock dropped.  It is also possible that
         * an overlapping map exists in the tree
         */
        if (ret == -EEXIST) {
                struct extent_map *existing;

                ret = 0;

                existing = search_extent_mapping(em_tree, start, len);

                trace_btrfs_handle_em_exist(fs_info, existing, em, start, len);

                /*
                 * existing will always be non-NULL, since there must be
                 * extent causing the -EEXIST.
                 */
                if (start >= existing->start &&
                    start < extent_map_end(existing)) {
                        free_extent_map(em);
                        *em_in = existing;
                        ret = 0;
                } else {
                        u64 orig_start = em->start;
                        u64 orig_len = em->len;

                        /*
                         * The existing extent map is the one nearest to
                         * the [start, start + len) range which overlaps
                         */
                        ret = merge_extent_mapping(em_tree, existing,
                                                   em, start);
                        if (ret) {
                                free_extent_map(em);
                                *em_in = NULL;
                                WARN_ONCE(ret,
"unexpected error %d: merge existing(start %llu len %llu) with em(start %llu len %llu)\n",
                                          ret, existing->start, existing->len,
                                          orig_start, orig_len);
                        }
                        free_extent_map(existing);
                }
        }

        ASSERT(ret == 0 || ret == -EEXIST);
        return ret;
}

/*
 * Drop all extent maps from a tree in the fastest possible way, rescheduling
 * if needed. This avoids searching the tree, from the root down to the first
 * extent map, before each deletion.
 */
static void drop_all_extent_maps_fast(struct extent_map_tree *tree)
{
        write_lock(&tree->lock);
        while (!RB_EMPTY_ROOT(&tree->map.rb_root)) {
                struct extent_map *em;
                struct rb_node *node;

                node = rb_first_cached(&tree->map);
                em = rb_entry(node, struct extent_map, rb_node);
                clear_bit(EXTENT_FLAG_PINNED, &em->flags);
                clear_bit(EXTENT_FLAG_LOGGING, &em->flags);
                remove_extent_mapping(tree, em);
                free_extent_map(em);
                cond_resched_rwlock_write(&tree->lock);
        }
        write_unlock(&tree->lock);
}

/*
 * Drop all extent maps in a given range.
 *
 * @inode:       The target inode.
 * @start:       Start offset of the range.
 * @end:         End offset of the range (inclusive value).
 * @skip_pinned: Indicate if pinned extent maps should be ignored or not.
 *
 * This drops all the extent maps that intersect the given range [@start, @end].
 * Extent maps that partially overlap the range and extend behind or beyond it,
 * are split.
 * The caller should have locked an appropriate file range in the inode's io
 * tree before calling this function.
 */
void btrfs_drop_extent_map_range(struct btrfs_inode *inode, u64 start, u64 end,
                                 bool skip_pinned)
{
        struct extent_map *split;
        struct extent_map *split2;
        struct extent_map *em;
        struct extent_map_tree *em_tree = &inode->extent_tree;
        u64 len = end - start + 1;

        WARN_ON(end < start);
        if (end == (u64)-1) {
                if (start == 0 && !skip_pinned) {
                        drop_all_extent_maps_fast(em_tree);
                        return;
                }
                len = (u64)-1;
        } else {
                /* Make end offset exclusive for use in the loop below. */
                end++;
        }

        /*
         * It's ok if we fail to allocate the extent maps, see the comment near
         * the bottom of the loop below. We only need two spare extent maps in
         * the worst case, where the first extent map that intersects our range
         * starts before the range and the last extent map that intersects our
         * range ends after our range (and they might be the same extent map),
         * because we need to split those two extent maps at the boundaries.
         */
        split = alloc_extent_map();
        split2 = alloc_extent_map();

        write_lock(&em_tree->lock);
        em = lookup_extent_mapping(em_tree, start, len);

        while (em) {
                /* extent_map_end() returns exclusive value (last byte + 1). */
                const u64 em_end = extent_map_end(em);
                struct extent_map *next_em = NULL;
                u64 gen;
                unsigned long flags;
                bool modified;
                bool compressed;

                if (em_end < end) {
                        next_em = next_extent_map(em);
                        if (next_em) {
                                if (next_em->start < end)
                                        refcount_inc(&next_em->refs);
                                else
                                        next_em = NULL;
                        }
                }

                if (skip_pinned && test_bit(EXTENT_FLAG_PINNED, &em->flags)) {
                        start = em_end;
                        goto next;
                }

                flags = em->flags;
                clear_bit(EXTENT_FLAG_PINNED, &em->flags);
                /*
                 * In case we split the extent map, we want to preserve the
                 * EXTENT_FLAG_LOGGING flag on our extent map, but we don't want
                 * it on the new extent maps.
                 */
                clear_bit(EXTENT_FLAG_LOGGING, &flags);
                modified = !list_empty(&em->list);

                /*
                 * The extent map does not cross our target range, so no need to
                 * split it, we can remove it directly.
                 */
                if (em->start >= start && em_end <= end)
                        goto remove_em;

                gen = em->generation;
                compressed = test_bit(EXTENT_FLAG_COMPRESSED, &em->flags);

                if (em->start < start) {
                        if (!split) {
                                split = split2;
                                split2 = NULL;
                                if (!split)
                                        goto remove_em;
                        }
                        split->start = em->start;
                        split->len = start - em->start;

                        if (em->block_start < EXTENT_MAP_LAST_BYTE) {
                                split->orig_start = em->orig_start;
                                split->block_start = em->block_start;

                                if (compressed)
                                        split->block_len = em->block_len;
                                else
                                        split->block_len = split->len;
                                split->orig_block_len = max(split->block_len,
                                                em->orig_block_len);
                                split->ram_bytes = em->ram_bytes;
                        } else {
                                split->orig_start = split->start;
                                split->block_len = 0;
                                split->block_start = em->block_start;
                                split->orig_block_len = 0;
                                split->ram_bytes = split->len;
                        }

                        split->generation = gen;
                        split->flags = flags;
                        split->compress_type = em->compress_type;
                        replace_extent_mapping(em_tree, em, split, modified);
                        free_extent_map(split);
                        split = split2;
                        split2 = NULL;
                }
                if (em_end > end) {
                        if (!split) {
                                split = split2;
                                split2 = NULL;
                                if (!split)
                                        goto remove_em;
                        }
                        split->start = end;
                        split->len = em_end - end;
                        split->block_start = em->block_start;
                        split->flags = flags;
                        split->compress_type = em->compress_type;
                        split->generation = gen;

                        if (em->block_start < EXTENT_MAP_LAST_BYTE) {
                                split->orig_block_len = max(em->block_len,
                                                    em->orig_block_len);

                                split->ram_bytes = em->ram_bytes;
                                if (compressed) {
                                        split->block_len = em->block_len;
                                        split->orig_start = em->orig_start;
                                } else {
                                        const u64 diff = start + len - em->start;

                                        split->block_len = split->len;
                                        split->block_start += diff;
                                        split->orig_start = em->orig_start;
                                }
                        } else {
                                split->ram_bytes = split->len;
                                split->orig_start = split->start;
                                split->block_len = 0;
                                split->orig_block_len = 0;
                        }

                        if (extent_map_in_tree(em)) {
                                replace_extent_mapping(em_tree, em, split,
                                                       modified);
                        } else {
                                int ret;

                                ret = add_extent_mapping(em_tree, split,
                                                         modified);
                                /* Logic error, shouldn't happen. */
                                ASSERT(ret == 0);
                                if (WARN_ON(ret != 0) && modified)
                                        btrfs_set_inode_full_sync(inode);
                        }
                        free_extent_map(split);
                        split = NULL;
                }
remove_em:
                if (extent_map_in_tree(em)) {
                        /*
                         * If the extent map is still in the tree it means that
                         * either of the following is true:
                         *
                         * 1) It fits entirely in our range (doesn't end beyond
                         *    it or starts before it);
                         *
                         * 2) It starts before our range and/or ends after our
                         *    range, and we were not able to allocate the extent
                         *    maps for split operations, @split and @split2.
                         *
                         * If we are at case 2) then we just remove the entire
                         * extent map - this is fine since if anyone needs it to
                         * access the subranges outside our range, will just
                         * load it again from the subvolume tree's file extent
                         * item. However if the extent map was in the list of
                         * modified extents, then we must mark the inode for a
                         * full fsync, otherwise a fast fsync will miss this
                         * extent if it's new and needs to be logged.
                         */
                        if ((em->start < start || em_end > end) && modified) {
                                ASSERT(!split);
                                btrfs_set_inode_full_sync(inode);
                        }
                        remove_extent_mapping(em_tree, em);
                }

                /*
                 * Once for the tree reference (we replaced or removed the
                 * extent map from the tree).
                 */
                free_extent_map(em);
next:
                /* Once for us (for our lookup reference). */
                free_extent_map(em);

                em = next_em;
        }

        write_unlock(&em_tree->lock);

        free_extent_map(split);
        free_extent_map(split2);
}

/*
 * Replace a range in the inode's extent map tree with a new extent map.
 *
 * @inode:      The target inode.
 * @new_em:     The new extent map to add to the inode's extent map tree.
 * @modified:   Indicate if the new extent map should be added to the list of
 *              modified extents (for fast fsync tracking).
 *
 * Drops all the extent maps in the inode's extent map tree that intersect the
 * range of the new extent map and adds the new extent map to the tree.
 * The caller should have locked an appropriate file range in the inode's io
 * tree before calling this function.
 */
int btrfs_replace_extent_map_range(struct btrfs_inode *inode,
                                   struct extent_map *new_em,
                                   bool modified)
{
        const u64 end = new_em->start + new_em->len - 1;
        struct extent_map_tree *tree = &inode->extent_tree;
        int ret;

        ASSERT(!extent_map_in_tree(new_em));

        /*
         * The caller has locked an appropriate file range in the inode's io
         * tree, but getting -EEXIST when adding the new extent map can still
         * happen in case there are extents that partially cover the range, and
         * this is due to two tasks operating on different parts of the extent.
         * See commit 18e83ac75bfe67 ("Btrfs: fix unexpected EEXIST from
         * btrfs_get_extent") for an example and details.
         */
        do {
                btrfs_drop_extent_map_range(inode, new_em->start, end, false);
                write_lock(&tree->lock);
                ret = add_extent_mapping(tree, new_em, modified);
                write_unlock(&tree->lock);
        } while (ret == -EEXIST);

        return ret;
}

/*
 * Split off the first pre bytes from the extent_map at [start, start + len],
 * and set the block_start for it to new_logical.
 *
 * This function is used when an ordered_extent needs to be split.
 */
int split_extent_map(struct btrfs_inode *inode, u64 start, u64 len, u64 pre,
                     u64 new_logical)
{
        struct extent_map_tree *em_tree = &inode->extent_tree;
        struct extent_map *em;
        struct extent_map *split_pre = NULL;
        struct extent_map *split_mid = NULL;
        int ret = 0;
        unsigned long flags;

        ASSERT(pre != 0);
        ASSERT(pre < len);

        split_pre = alloc_extent_map();
        if (!split_pre)
                return -ENOMEM;
        split_mid = alloc_extent_map();
        if (!split_mid) {
                ret = -ENOMEM;
                goto out_free_pre;
        }

        lock_extent(&inode->io_tree, start, start + len - 1, NULL);
        write_lock(&em_tree->lock);
        em = lookup_extent_mapping(em_tree, start, len);
        if (!em) {
                ret = -EIO;
                goto out_unlock;
        }

        ASSERT(em->len == len);
        ASSERT(!test_bit(EXTENT_FLAG_COMPRESSED, &em->flags));
        ASSERT(em->block_start < EXTENT_MAP_LAST_BYTE);
        ASSERT(test_bit(EXTENT_FLAG_PINNED, &em->flags));
        ASSERT(!test_bit(EXTENT_FLAG_LOGGING, &em->flags));
        ASSERT(!list_empty(&em->list));

        flags = em->flags;
        clear_bit(EXTENT_FLAG_PINNED, &em->flags);

        /* First, replace the em with a new extent_map starting from * em->start */
        split_pre->start = em->start;
        split_pre->len = pre;
        split_pre->orig_start = split_pre->start;
        split_pre->block_start = new_logical;
        split_pre->block_len = split_pre->len;
        split_pre->orig_block_len = split_pre->block_len;
        split_pre->ram_bytes = split_pre->len;
        split_pre->flags = flags;
        split_pre->compress_type = em->compress_type;
        split_pre->generation = em->generation;

        replace_extent_mapping(em_tree, em, split_pre, 1);

        /*
         * Now we only have an extent_map at:
         *     [em->start, em->start + pre]
         */

        /* Insert the middle extent_map. */
        split_mid->start = em->start + pre;
        split_mid->len = em->len - pre;
        split_mid->orig_start = split_mid->start;
        split_mid->block_start = em->block_start + pre;
        split_mid->block_len = split_mid->len;
        split_mid->orig_block_len = split_mid->block_len;
        split_mid->ram_bytes = split_mid->len;
        split_mid->flags = flags;
        split_mid->compress_type = em->compress_type;
        split_mid->generation = em->generation;
        add_extent_mapping(em_tree, split_mid, 1);

        /* Once for us */
        free_extent_map(em);
        /* Once for the tree */
        free_extent_map(em);

out_unlock:
        write_unlock(&em_tree->lock);
        unlock_extent(&inode->io_tree, start, start + len - 1, NULL);
        free_extent_map(split_mid);
out_free_pre:
        free_extent_map(split_pre);
        return ret;
}