Merge tag 'soc-late-6.9' of git://git.kernel.org/pub/scm/linux/kernel/git/soc/soc

[thirdparty/linux.git] / fs / bcachefs / snapshot.c

// SPDX-License-Identifier: GPL-2.0

#include "bcachefs.h"
#include "bkey_buf.h"
#include "btree_key_cache.h"
#include "btree_update.h"
#include "buckets.h"
#include "errcode.h"
#include "error.h"
#include "fs.h"
#include "snapshot.h"

#include <linux/random.h>

/*
 * Snapshot trees:
 *
 * Keys in BTREE_ID_snapshot_trees identify a whole tree of snapshot nodes; they
 * exist to provide a stable identifier for the whole lifetime of a snapshot
 * tree.
 */

void bch2_snapshot_tree_to_text(struct printbuf *out, struct bch_fs *c,
                                struct bkey_s_c k)
{
        struct bkey_s_c_snapshot_tree t = bkey_s_c_to_snapshot_tree(k);

        prt_printf(out, "subvol %u root snapshot %u",
                   le32_to_cpu(t.v->master_subvol),
                   le32_to_cpu(t.v->root_snapshot));
}

int bch2_snapshot_tree_invalid(struct bch_fs *c, struct bkey_s_c k,
                               enum bkey_invalid_flags flags,
                               struct printbuf *err)
{
        int ret = 0;

        bkey_fsck_err_on(bkey_gt(k.k->p, POS(0, U32_MAX)) ||
                         bkey_lt(k.k->p, POS(0, 1)), c, err,
                         snapshot_tree_pos_bad,
                         "bad pos");
fsck_err:
        return ret;
}

int bch2_snapshot_tree_lookup(struct btree_trans *trans, u32 id,
                              struct bch_snapshot_tree *s)
{
        int ret = bch2_bkey_get_val_typed(trans, BTREE_ID_snapshot_trees, POS(0, id),
                                          BTREE_ITER_WITH_UPDATES, snapshot_tree, s);

        if (bch2_err_matches(ret, ENOENT))
                ret = -BCH_ERR_ENOENT_snapshot_tree;
        return ret;
}

struct bkey_i_snapshot_tree *
__bch2_snapshot_tree_create(struct btree_trans *trans)
{
        struct btree_iter iter;
        int ret = bch2_bkey_get_empty_slot(trans, &iter,
                        BTREE_ID_snapshot_trees, POS(0, U32_MAX));
        struct bkey_i_snapshot_tree *s_t;

        if (ret == -BCH_ERR_ENOSPC_btree_slot)
                ret = -BCH_ERR_ENOSPC_snapshot_tree;
        if (ret)
                return ERR_PTR(ret);

        s_t = bch2_bkey_alloc(trans, &iter, 0, snapshot_tree);
        ret = PTR_ERR_OR_ZERO(s_t);
        bch2_trans_iter_exit(trans, &iter);
        return ret ? ERR_PTR(ret) : s_t;
}

static int bch2_snapshot_tree_create(struct btree_trans *trans,
                                u32 root_id, u32 subvol_id, u32 *tree_id)
{
        struct bkey_i_snapshot_tree *n_tree =
                __bch2_snapshot_tree_create(trans);

        if (IS_ERR(n_tree))
                return PTR_ERR(n_tree);

        n_tree->v.master_subvol = cpu_to_le32(subvol_id);
        n_tree->v.root_snapshot = cpu_to_le32(root_id);
        *tree_id = n_tree->k.p.offset;
        return 0;
}

/* Snapshot nodes: */

static bool bch2_snapshot_is_ancestor_early(struct bch_fs *c, u32 id, u32 ancestor)
{
        struct snapshot_table *t;

        rcu_read_lock();
        t = rcu_dereference(c->snapshots);

        while (id && id < ancestor)
                id = __snapshot_t(t, id)->parent;
        rcu_read_unlock();

        return id == ancestor;
}

static inline u32 get_ancestor_below(struct snapshot_table *t, u32 id, u32 ancestor)
{
        const struct snapshot_t *s = __snapshot_t(t, id);

        if (s->skip[2] <= ancestor)
                return s->skip[2];
        if (s->skip[1] <= ancestor)
                return s->skip[1];
        if (s->skip[0] <= ancestor)
                return s->skip[0];
        return s->parent;
}

bool __bch2_snapshot_is_ancestor(struct bch_fs *c, u32 id, u32 ancestor)
{
        struct snapshot_table *t;
        bool ret;

        EBUG_ON(c->recovery_pass_done <= BCH_RECOVERY_PASS_check_snapshots);

        rcu_read_lock();
        t = rcu_dereference(c->snapshots);

        while (id && id < ancestor - IS_ANCESTOR_BITMAP)
                id = get_ancestor_below(t, id, ancestor);

        if (id && id < ancestor) {
                ret = test_bit(ancestor - id - 1, __snapshot_t(t, id)->is_ancestor);

                EBUG_ON(ret != bch2_snapshot_is_ancestor_early(c, id, ancestor));
        } else {
                ret = id == ancestor;
        }

        rcu_read_unlock();

        return ret;
}

static noinline struct snapshot_t *__snapshot_t_mut(struct bch_fs *c, u32 id)
{
        size_t idx = U32_MAX - id;
        size_t new_size;
        struct snapshot_table *new, *old;

        new_size = max(16UL, roundup_pow_of_two(idx + 1));

        new = kvzalloc(struct_size(new, s, new_size), GFP_KERNEL);
        if (!new)
                return NULL;

        old = rcu_dereference_protected(c->snapshots, true);
        if (old)
                memcpy(new->s,
                       rcu_dereference_protected(c->snapshots, true)->s,
                       sizeof(new->s[0]) * c->snapshot_table_size);

        rcu_assign_pointer(c->snapshots, new);
        c->snapshot_table_size = new_size;
        kvfree_rcu_mightsleep(old);

        return &rcu_dereference_protected(c->snapshots, true)->s[idx];
}

static inline struct snapshot_t *snapshot_t_mut(struct bch_fs *c, u32 id)
{
        size_t idx = U32_MAX - id;

        lockdep_assert_held(&c->snapshot_table_lock);

        if (likely(idx < c->snapshot_table_size))
                return &rcu_dereference_protected(c->snapshots, true)->s[idx];

        return __snapshot_t_mut(c, id);
}

void bch2_snapshot_to_text(struct printbuf *out, struct bch_fs *c,
                           struct bkey_s_c k)
{
        struct bkey_s_c_snapshot s = bkey_s_c_to_snapshot(k);

        prt_printf(out, "is_subvol %llu deleted %llu parent %10u children %10u %10u subvol %u tree %u",
               BCH_SNAPSHOT_SUBVOL(s.v),
               BCH_SNAPSHOT_DELETED(s.v),
               le32_to_cpu(s.v->parent),
               le32_to_cpu(s.v->children[0]),
               le32_to_cpu(s.v->children[1]),
               le32_to_cpu(s.v->subvol),
               le32_to_cpu(s.v->tree));

        if (bkey_val_bytes(k.k) > offsetof(struct bch_snapshot, depth))
                prt_printf(out, " depth %u skiplist %u %u %u",
                           le32_to_cpu(s.v->depth),
                           le32_to_cpu(s.v->skip[0]),
                           le32_to_cpu(s.v->skip[1]),
                           le32_to_cpu(s.v->skip[2]));
}

int bch2_snapshot_invalid(struct bch_fs *c, struct bkey_s_c k,
                          enum bkey_invalid_flags flags,
                          struct printbuf *err)
{
        struct bkey_s_c_snapshot s;
        u32 i, id;
        int ret = 0;

        bkey_fsck_err_on(bkey_gt(k.k->p, POS(0, U32_MAX)) ||
                         bkey_lt(k.k->p, POS(0, 1)), c, err,
                         snapshot_pos_bad,
                         "bad pos");

        s = bkey_s_c_to_snapshot(k);

        id = le32_to_cpu(s.v->parent);
        bkey_fsck_err_on(id && id <= k.k->p.offset, c, err,
                         snapshot_parent_bad,
                         "bad parent node (%u <= %llu)",
                         id, k.k->p.offset);

        bkey_fsck_err_on(le32_to_cpu(s.v->children[0]) < le32_to_cpu(s.v->children[1]), c, err,
                         snapshot_children_not_normalized,
                         "children not normalized");

        bkey_fsck_err_on(s.v->children[0] && s.v->children[0] == s.v->children[1], c, err,
                         snapshot_child_duplicate,
                         "duplicate child nodes");

        for (i = 0; i < 2; i++) {
                id = le32_to_cpu(s.v->children[i]);

                bkey_fsck_err_on(id >= k.k->p.offset, c, err,
                                 snapshot_child_bad,
                                 "bad child node (%u >= %llu)",
                                 id, k.k->p.offset);
        }

        if (bkey_val_bytes(k.k) > offsetof(struct bch_snapshot, skip)) {
                bkey_fsck_err_on(le32_to_cpu(s.v->skip[0]) > le32_to_cpu(s.v->skip[1]) ||
                                 le32_to_cpu(s.v->skip[1]) > le32_to_cpu(s.v->skip[2]), c, err,
                                 snapshot_skiplist_not_normalized,
                                 "skiplist not normalized");

                for (i = 0; i < ARRAY_SIZE(s.v->skip); i++) {
                        id = le32_to_cpu(s.v->skip[i]);

                        bkey_fsck_err_on(id && id < le32_to_cpu(s.v->parent), c, err,
                                         snapshot_skiplist_bad,
                                         "bad skiplist node %u", id);
                }
        }
fsck_err:
        return ret;
}

static void __set_is_ancestor_bitmap(struct bch_fs *c, u32 id)
{
        struct snapshot_t *t = snapshot_t_mut(c, id);
        u32 parent = id;

        while ((parent = bch2_snapshot_parent_early(c, parent)) &&
               parent - id - 1 < IS_ANCESTOR_BITMAP)
                __set_bit(parent - id - 1, t->is_ancestor);
}

static void set_is_ancestor_bitmap(struct bch_fs *c, u32 id)
{
        mutex_lock(&c->snapshot_table_lock);
        __set_is_ancestor_bitmap(c, id);
        mutex_unlock(&c->snapshot_table_lock);
}

static int __bch2_mark_snapshot(struct btree_trans *trans,
                       enum btree_id btree, unsigned level,
                       struct bkey_s_c old, struct bkey_s_c new,
                       unsigned flags)
{
        struct bch_fs *c = trans->c;
        struct snapshot_t *t;
        u32 id = new.k->p.offset;
        int ret = 0;

        mutex_lock(&c->snapshot_table_lock);

        t = snapshot_t_mut(c, id);
        if (!t) {
                ret = -BCH_ERR_ENOMEM_mark_snapshot;
                goto err;
        }

        if (new.k->type == KEY_TYPE_snapshot) {
                struct bkey_s_c_snapshot s = bkey_s_c_to_snapshot(new);

                t->parent       = le32_to_cpu(s.v->parent);
                t->children[0]  = le32_to_cpu(s.v->children[0]);
                t->children[1]  = le32_to_cpu(s.v->children[1]);
                t->subvol       = BCH_SNAPSHOT_SUBVOL(s.v) ? le32_to_cpu(s.v->subvol) : 0;
                t->tree         = le32_to_cpu(s.v->tree);

                if (bkey_val_bytes(s.k) > offsetof(struct bch_snapshot, depth)) {
                        t->depth        = le32_to_cpu(s.v->depth);
                        t->skip[0]      = le32_to_cpu(s.v->skip[0]);
                        t->skip[1]      = le32_to_cpu(s.v->skip[1]);
                        t->skip[2]      = le32_to_cpu(s.v->skip[2]);
                } else {
                        t->depth        = 0;
                        t->skip[0]      = 0;
                        t->skip[1]      = 0;
                        t->skip[2]      = 0;
                }

                __set_is_ancestor_bitmap(c, id);

                if (BCH_SNAPSHOT_DELETED(s.v)) {
                        set_bit(BCH_FS_need_delete_dead_snapshots, &c->flags);
                        if (c->curr_recovery_pass > BCH_RECOVERY_PASS_delete_dead_snapshots)
                                bch2_delete_dead_snapshots_async(c);
                }
        } else {
                memset(t, 0, sizeof(*t));
        }
err:
        mutex_unlock(&c->snapshot_table_lock);
        return ret;
}

int bch2_mark_snapshot(struct btree_trans *trans,
                       enum btree_id btree, unsigned level,
                       struct bkey_s_c old, struct bkey_s new,
                       unsigned flags)
{
        return __bch2_mark_snapshot(trans, btree, level, old, new.s_c, flags);
}

int bch2_snapshot_lookup(struct btree_trans *trans, u32 id,
                         struct bch_snapshot *s)
{
        return bch2_bkey_get_val_typed(trans, BTREE_ID_snapshots, POS(0, id),
                                       BTREE_ITER_WITH_UPDATES, snapshot, s);
}

static int bch2_snapshot_live(struct btree_trans *trans, u32 id)
{
        struct bch_snapshot v;
        int ret;

        if (!id)
                return 0;

        ret = bch2_snapshot_lookup(trans, id, &v);
        if (bch2_err_matches(ret, ENOENT))
                bch_err(trans->c, "snapshot node %u not found", id);
        if (ret)
                return ret;

        return !BCH_SNAPSHOT_DELETED(&v);
}

/*
 * If @k is a snapshot with just one live child, it's part of a linear chain,
 * which we consider to be an equivalence class: and then after snapshot
 * deletion cleanup, there should only be a single key at a given position in
 * this equivalence class.
 *
 * This sets the equivalence class of @k to be the child's equivalence class, if
 * it's part of such a linear chain: this correctly sets equivalence classes on
 * startup if we run leaf to root (i.e. in natural key order).
 */
static int bch2_snapshot_set_equiv(struct btree_trans *trans, struct bkey_s_c k)
{
        struct bch_fs *c = trans->c;
        unsigned i, nr_live = 0, live_idx = 0;
        struct bkey_s_c_snapshot snap;
        u32 id = k.k->p.offset, child[2];

        if (k.k->type != KEY_TYPE_snapshot)
                return 0;

        snap = bkey_s_c_to_snapshot(k);

        child[0] = le32_to_cpu(snap.v->children[0]);
        child[1] = le32_to_cpu(snap.v->children[1]);

        for (i = 0; i < 2; i++) {
                int ret = bch2_snapshot_live(trans, child[i]);

                if (ret < 0)
                        return ret;

                if (ret)
                        live_idx = i;
                nr_live += ret;
        }

        mutex_lock(&c->snapshot_table_lock);

        snapshot_t_mut(c, id)->equiv = nr_live == 1
                ? snapshot_t_mut(c, child[live_idx])->equiv
                : id;

        mutex_unlock(&c->snapshot_table_lock);

        return 0;
}

/* fsck: */

static u32 bch2_snapshot_child(struct bch_fs *c, u32 id, unsigned child)
{
        return snapshot_t(c, id)->children[child];
}

static u32 bch2_snapshot_left_child(struct bch_fs *c, u32 id)
{
        return bch2_snapshot_child(c, id, 0);
}

static u32 bch2_snapshot_right_child(struct bch_fs *c, u32 id)
{
        return bch2_snapshot_child(c, id, 1);
}

static u32 bch2_snapshot_tree_next(struct bch_fs *c, u32 id)
{
        u32 n, parent;

        n = bch2_snapshot_left_child(c, id);
        if (n)
                return n;

        while ((parent = bch2_snapshot_parent(c, id))) {
                n = bch2_snapshot_right_child(c, parent);
                if (n && n != id)
                        return n;
                id = parent;
        }

        return 0;
}

static u32 bch2_snapshot_tree_oldest_subvol(struct bch_fs *c, u32 snapshot_root)
{
        u32 id = snapshot_root;
        u32 subvol = 0, s;

        while (id) {
                s = snapshot_t(c, id)->subvol;

                if (s && (!subvol || s < subvol))
                        subvol = s;

                id = bch2_snapshot_tree_next(c, id);
        }

        return subvol;
}

static int bch2_snapshot_tree_master_subvol(struct btree_trans *trans,
                                            u32 snapshot_root, u32 *subvol_id)
{
        struct bch_fs *c = trans->c;
        struct btree_iter iter;
        struct bkey_s_c k;
        bool found = false;
        int ret;

        for_each_btree_key_norestart(trans, iter, BTREE_ID_subvolumes, POS_MIN,
                                     0, k, ret) {
                if (k.k->type != KEY_TYPE_subvolume)
                        continue;

                struct bkey_s_c_subvolume s = bkey_s_c_to_subvolume(k);
                if (!bch2_snapshot_is_ancestor(c, le32_to_cpu(s.v->snapshot), snapshot_root))
                        continue;
                if (!BCH_SUBVOLUME_SNAP(s.v)) {
                        *subvol_id = s.k->p.offset;
                        found = true;
                        break;
                }
        }

        bch2_trans_iter_exit(trans, &iter);

        if (!ret && !found) {
                struct bkey_i_subvolume *u;

                *subvol_id = bch2_snapshot_tree_oldest_subvol(c, snapshot_root);

                u = bch2_bkey_get_mut_typed(trans, &iter,
                                            BTREE_ID_subvolumes, POS(0, *subvol_id),
                                            0, subvolume);
                ret = PTR_ERR_OR_ZERO(u);
                if (ret)
                        return ret;

                SET_BCH_SUBVOLUME_SNAP(&u->v, false);
        }

        return ret;
}

static int check_snapshot_tree(struct btree_trans *trans,
                               struct btree_iter *iter,
                               struct bkey_s_c k)
{
        struct bch_fs *c = trans->c;
        struct bkey_s_c_snapshot_tree st;
        struct bch_snapshot s;
        struct bch_subvolume subvol;
        struct printbuf buf = PRINTBUF;
        u32 root_id;
        int ret;

        if (k.k->type != KEY_TYPE_snapshot_tree)
                return 0;

        st = bkey_s_c_to_snapshot_tree(k);
        root_id = le32_to_cpu(st.v->root_snapshot);

        ret = bch2_snapshot_lookup(trans, root_id, &s);
        if (ret && !bch2_err_matches(ret, ENOENT))
                goto err;

        if (fsck_err_on(ret ||
                        root_id != bch2_snapshot_root(c, root_id) ||
                        st.k->p.offset != le32_to_cpu(s.tree),
                        c, snapshot_tree_to_missing_snapshot,
                        "snapshot tree points to missing/incorrect snapshot:\n  %s",
                        (bch2_bkey_val_to_text(&buf, c, st.s_c), buf.buf))) {
                ret = bch2_btree_delete_at(trans, iter, 0);
                goto err;
        }

        ret = bch2_subvolume_get(trans, le32_to_cpu(st.v->master_subvol),
                                 false, 0, &subvol);
        if (ret && !bch2_err_matches(ret, ENOENT))
                goto err;

        if (fsck_err_on(ret,
                        c, snapshot_tree_to_missing_subvol,
                        "snapshot tree points to missing subvolume:\n  %s",
                        (printbuf_reset(&buf),
                         bch2_bkey_val_to_text(&buf, c, st.s_c), buf.buf)) ||
            fsck_err_on(!bch2_snapshot_is_ancestor_early(c,
                                                le32_to_cpu(subvol.snapshot),
                                                root_id),
                        c, snapshot_tree_to_wrong_subvol,
                        "snapshot tree points to subvolume that does not point to snapshot in this tree:\n  %s",
                        (printbuf_reset(&buf),
                         bch2_bkey_val_to_text(&buf, c, st.s_c), buf.buf)) ||
            fsck_err_on(BCH_SUBVOLUME_SNAP(&subvol),
                        c, snapshot_tree_to_snapshot_subvol,
                        "snapshot tree points to snapshot subvolume:\n  %s",
                        (printbuf_reset(&buf),
                         bch2_bkey_val_to_text(&buf, c, st.s_c), buf.buf))) {
                struct bkey_i_snapshot_tree *u;
                u32 subvol_id;

                ret = bch2_snapshot_tree_master_subvol(trans, root_id, &subvol_id);
                if (ret)
                        goto err;

                u = bch2_bkey_make_mut_typed(trans, iter, &k, 0, snapshot_tree);
                ret = PTR_ERR_OR_ZERO(u);
                if (ret)
                        goto err;

                u->v.master_subvol = cpu_to_le32(subvol_id);
                st = snapshot_tree_i_to_s_c(u);
        }
err:
fsck_err:
        printbuf_exit(&buf);
        return ret;
}

/*
 * For each snapshot_tree, make sure it points to the root of a snapshot tree
 * and that snapshot entry points back to it, or delete it.
 *
 * And, make sure it points to a subvolume within that snapshot tree, or correct
 * it to point to the oldest subvolume within that snapshot tree.
 */
int bch2_check_snapshot_trees(struct bch_fs *c)
{
        int ret = bch2_trans_run(c,
                for_each_btree_key_commit(trans, iter,
                        BTREE_ID_snapshot_trees, POS_MIN,
                        BTREE_ITER_PREFETCH, k,
                        NULL, NULL, BCH_TRANS_COMMIT_no_enospc,
                check_snapshot_tree(trans, &iter, k)));
        bch_err_fn(c, ret);
        return ret;
}

/*
 * Look up snapshot tree for @tree_id and find root,
 * make sure @snap_id is a descendent:
 */
static int snapshot_tree_ptr_good(struct btree_trans *trans,
                                  u32 snap_id, u32 tree_id)
{
        struct bch_snapshot_tree s_t;
        int ret = bch2_snapshot_tree_lookup(trans, tree_id, &s_t);

        if (bch2_err_matches(ret, ENOENT))
                return 0;
        if (ret)
                return ret;

        return bch2_snapshot_is_ancestor_early(trans->c, snap_id, le32_to_cpu(s_t.root_snapshot));
}

u32 bch2_snapshot_skiplist_get(struct bch_fs *c, u32 id)
{
        const struct snapshot_t *s;

        if (!id)
                return 0;

        rcu_read_lock();
        s = snapshot_t(c, id);
        if (s->parent)
                id = bch2_snapshot_nth_parent(c, id, get_random_u32_below(s->depth));
        rcu_read_unlock();

        return id;
}

static int snapshot_skiplist_good(struct btree_trans *trans, u32 id, struct bch_snapshot s)
{
        unsigned i;

        for (i = 0; i < 3; i++)
                if (!s.parent) {
                        if (s.skip[i])
                                return false;
                } else {
                        if (!bch2_snapshot_is_ancestor_early(trans->c, id, le32_to_cpu(s.skip[i])))
                                return false;
                }

        return true;
}

/*
 * snapshot_tree pointer was incorrect: look up root snapshot node, make sure
 * its snapshot_tree pointer is correct (allocate new one if necessary), then
 * update this node's pointer to root node's pointer:
 */
static int snapshot_tree_ptr_repair(struct btree_trans *trans,
                                    struct btree_iter *iter,
                                    struct bkey_s_c k,
                                    struct bch_snapshot *s)
{
        struct bch_fs *c = trans->c;
        struct btree_iter root_iter;
        struct bch_snapshot_tree s_t;
        struct bkey_s_c_snapshot root;
        struct bkey_i_snapshot *u;
        u32 root_id = bch2_snapshot_root(c, k.k->p.offset), tree_id;
        int ret;

        root = bch2_bkey_get_iter_typed(trans, &root_iter,
                               BTREE_ID_snapshots, POS(0, root_id),
                               BTREE_ITER_WITH_UPDATES, snapshot);
        ret = bkey_err(root);
        if (ret)
                goto err;

        tree_id = le32_to_cpu(root.v->tree);

        ret = bch2_snapshot_tree_lookup(trans, tree_id, &s_t);
        if (ret && !bch2_err_matches(ret, ENOENT))
                return ret;

        if (ret || le32_to_cpu(s_t.root_snapshot) != root_id) {
                u = bch2_bkey_make_mut_typed(trans, &root_iter, &root.s_c, 0, snapshot);
                ret =   PTR_ERR_OR_ZERO(u) ?:
                        bch2_snapshot_tree_create(trans, root_id,
                                bch2_snapshot_tree_oldest_subvol(c, root_id),
                                &tree_id);
                if (ret)
                        goto err;

                u->v.tree = cpu_to_le32(tree_id);
                if (k.k->p.offset == root_id)
                        *s = u->v;
        }

        if (k.k->p.offset != root_id) {
                u = bch2_bkey_make_mut_typed(trans, iter, &k, 0, snapshot);
                ret = PTR_ERR_OR_ZERO(u);
                if (ret)
                        goto err;

                u->v.tree = cpu_to_le32(tree_id);
                *s = u->v;
        }
err:
        bch2_trans_iter_exit(trans, &root_iter);
        return ret;
}

static int check_snapshot(struct btree_trans *trans,
                          struct btree_iter *iter,
                          struct bkey_s_c k)
{
        struct bch_fs *c = trans->c;
        struct bch_snapshot s;
        struct bch_subvolume subvol;
        struct bch_snapshot v;
        struct bkey_i_snapshot *u;
        u32 parent_id = bch2_snapshot_parent_early(c, k.k->p.offset);
        u32 real_depth;
        struct printbuf buf = PRINTBUF;
        bool should_have_subvol;
        u32 i, id;
        int ret = 0;

        if (k.k->type != KEY_TYPE_snapshot)
                return 0;

        memset(&s, 0, sizeof(s));
        memcpy(&s, k.v, min(sizeof(s), bkey_val_bytes(k.k)));

        id = le32_to_cpu(s.parent);
        if (id) {
                ret = bch2_snapshot_lookup(trans, id, &v);
                if (bch2_err_matches(ret, ENOENT))
                        bch_err(c, "snapshot with nonexistent parent:\n  %s",
                                (bch2_bkey_val_to_text(&buf, c, k), buf.buf));
                if (ret)
                        goto err;

                if (le32_to_cpu(v.children[0]) != k.k->p.offset &&
                    le32_to_cpu(v.children[1]) != k.k->p.offset) {
                        bch_err(c, "snapshot parent %u missing pointer to child %llu",
                                id, k.k->p.offset);
                        ret = -EINVAL;
                        goto err;
                }
        }

        for (i = 0; i < 2 && s.children[i]; i++) {
                id = le32_to_cpu(s.children[i]);

                ret = bch2_snapshot_lookup(trans, id, &v);
                if (bch2_err_matches(ret, ENOENT))
                        bch_err(c, "snapshot node %llu has nonexistent child %u",
                                k.k->p.offset, id);
                if (ret)
                        goto err;

                if (le32_to_cpu(v.parent) != k.k->p.offset) {
                        bch_err(c, "snapshot child %u has wrong parent (got %u should be %llu)",
                                id, le32_to_cpu(v.parent), k.k->p.offset);
                        ret = -EINVAL;
                        goto err;
                }
        }

        should_have_subvol = BCH_SNAPSHOT_SUBVOL(&s) &&
                !BCH_SNAPSHOT_DELETED(&s);

        if (should_have_subvol) {
                id = le32_to_cpu(s.subvol);
                ret = bch2_subvolume_get(trans, id, 0, false, &subvol);
                if (bch2_err_matches(ret, ENOENT))
                        bch_err(c, "snapshot points to nonexistent subvolume:\n  %s",
                                (bch2_bkey_val_to_text(&buf, c, k), buf.buf));
                if (ret)
                        goto err;

                if (BCH_SNAPSHOT_SUBVOL(&s) != (le32_to_cpu(subvol.snapshot) == k.k->p.offset)) {
                        bch_err(c, "snapshot node %llu has wrong BCH_SNAPSHOT_SUBVOL",
                                k.k->p.offset);
                        ret = -EINVAL;
                        goto err;
                }
        } else {
                if (fsck_err_on(s.subvol,
                                c, snapshot_should_not_have_subvol,
                                "snapshot should not point to subvol:\n  %s",
                                (bch2_bkey_val_to_text(&buf, c, k), buf.buf))) {
                        u = bch2_bkey_make_mut_typed(trans, iter, &k, 0, snapshot);
                        ret = PTR_ERR_OR_ZERO(u);
                        if (ret)
                                goto err;

                        u->v.subvol = 0;
                        s = u->v;
                }
        }

        ret = snapshot_tree_ptr_good(trans, k.k->p.offset, le32_to_cpu(s.tree));
        if (ret < 0)
                goto err;

        if (fsck_err_on(!ret, c, snapshot_to_bad_snapshot_tree,
                        "snapshot points to missing/incorrect tree:\n  %s",
                        (bch2_bkey_val_to_text(&buf, c, k), buf.buf))) {
                ret = snapshot_tree_ptr_repair(trans, iter, k, &s);
                if (ret)
                        goto err;
        }
        ret = 0;

        real_depth = bch2_snapshot_depth(c, parent_id);

        if (fsck_err_on(le32_to_cpu(s.depth) != real_depth,
                        c, snapshot_bad_depth,
                        "snapshot with incorrect depth field, should be %u:\n  %s",
                        real_depth, (bch2_bkey_val_to_text(&buf, c, k), buf.buf))) {
                u = bch2_bkey_make_mut_typed(trans, iter, &k, 0, snapshot);
                ret = PTR_ERR_OR_ZERO(u);
                if (ret)
                        goto err;

                u->v.depth = cpu_to_le32(real_depth);
                s = u->v;
        }

        ret = snapshot_skiplist_good(trans, k.k->p.offset, s);
        if (ret < 0)
                goto err;

        if (fsck_err_on(!ret, c, snapshot_bad_skiplist,
                        "snapshot with bad skiplist field:\n  %s",
                        (bch2_bkey_val_to_text(&buf, c, k), buf.buf))) {
                u = bch2_bkey_make_mut_typed(trans, iter, &k, 0, snapshot);
                ret = PTR_ERR_OR_ZERO(u);
                if (ret)
                        goto err;

                for (i = 0; i < ARRAY_SIZE(u->v.skip); i++)
                        u->v.skip[i] = cpu_to_le32(bch2_snapshot_skiplist_get(c, parent_id));

                bubble_sort(u->v.skip, ARRAY_SIZE(u->v.skip), cmp_le32);
                s = u->v;
        }
        ret = 0;
err:
fsck_err:
        printbuf_exit(&buf);
        return ret;
}

int bch2_check_snapshots(struct bch_fs *c)
{
        /*
         * We iterate backwards as checking/fixing the depth field requires that
         * the parent's depth already be correct:
         */
        int ret = bch2_trans_run(c,
                for_each_btree_key_reverse_commit(trans, iter,
                                BTREE_ID_snapshots, POS_MAX,
                                BTREE_ITER_PREFETCH, k,
                                NULL, NULL, BCH_TRANS_COMMIT_no_enospc,
                        check_snapshot(trans, &iter, k)));
        bch_err_fn(c, ret);
        return ret;
}

/*
 * Mark a snapshot as deleted, for future cleanup:
 */
int bch2_snapshot_node_set_deleted(struct btree_trans *trans, u32 id)
{
        struct btree_iter iter;
        struct bkey_i_snapshot *s;
        int ret = 0;

        s = bch2_bkey_get_mut_typed(trans, &iter,
                                    BTREE_ID_snapshots, POS(0, id),
                                    0, snapshot);
        ret = PTR_ERR_OR_ZERO(s);
        if (unlikely(ret)) {
                bch2_fs_inconsistent_on(bch2_err_matches(ret, ENOENT),
                                        trans->c, "missing snapshot %u", id);
                return ret;
        }

        /* already deleted? */
        if (BCH_SNAPSHOT_DELETED(&s->v))
                goto err;

        SET_BCH_SNAPSHOT_DELETED(&s->v, true);
        SET_BCH_SNAPSHOT_SUBVOL(&s->v, false);
        s->v.subvol = 0;
err:
        bch2_trans_iter_exit(trans, &iter);
        return ret;
}

static inline void normalize_snapshot_child_pointers(struct bch_snapshot *s)
{
        if (le32_to_cpu(s->children[0]) < le32_to_cpu(s->children[1]))
                swap(s->children[0], s->children[1]);
}

static int bch2_snapshot_node_delete(struct btree_trans *trans, u32 id)
{
        struct bch_fs *c = trans->c;
        struct btree_iter iter, p_iter = (struct btree_iter) { NULL };
        struct btree_iter c_iter = (struct btree_iter) { NULL };
        struct btree_iter tree_iter = (struct btree_iter) { NULL };
        struct bkey_s_c_snapshot s;
        u32 parent_id, child_id;
        unsigned i;
        int ret = 0;

        s = bch2_bkey_get_iter_typed(trans, &iter, BTREE_ID_snapshots, POS(0, id),
                                     BTREE_ITER_INTENT, snapshot);
        ret = bkey_err(s);
        bch2_fs_inconsistent_on(bch2_err_matches(ret, ENOENT), c,
                                "missing snapshot %u", id);

        if (ret)
                goto err;

        BUG_ON(s.v->children[1]);

        parent_id = le32_to_cpu(s.v->parent);
        child_id = le32_to_cpu(s.v->children[0]);

        if (parent_id) {
                struct bkey_i_snapshot *parent;

                parent = bch2_bkey_get_mut_typed(trans, &p_iter,
                                     BTREE_ID_snapshots, POS(0, parent_id),
                                     0, snapshot);
                ret = PTR_ERR_OR_ZERO(parent);
                bch2_fs_inconsistent_on(bch2_err_matches(ret, ENOENT), c,
                                        "missing snapshot %u", parent_id);
                if (unlikely(ret))
                        goto err;

                /* find entry in parent->children for node being deleted */
                for (i = 0; i < 2; i++)
                        if (le32_to_cpu(parent->v.children[i]) == id)
                                break;

                if (bch2_fs_inconsistent_on(i == 2, c,
                                        "snapshot %u missing child pointer to %u",
                                        parent_id, id))
                        goto err;

                parent->v.children[i] = cpu_to_le32(child_id);

                normalize_snapshot_child_pointers(&parent->v);
        }

        if (child_id) {
                struct bkey_i_snapshot *child;

                child = bch2_bkey_get_mut_typed(trans, &c_iter,
                                     BTREE_ID_snapshots, POS(0, child_id),
                                     0, snapshot);
                ret = PTR_ERR_OR_ZERO(child);
                bch2_fs_inconsistent_on(bch2_err_matches(ret, ENOENT), c,
                                        "missing snapshot %u", child_id);
                if (unlikely(ret))
                        goto err;

                child->v.parent = cpu_to_le32(parent_id);

                if (!child->v.parent) {
                        child->v.skip[0] = 0;
                        child->v.skip[1] = 0;
                        child->v.skip[2] = 0;
                }
        }

        if (!parent_id) {
                /*
                 * We're deleting the root of a snapshot tree: update the
                 * snapshot_tree entry to point to the new root, or delete it if
                 * this is the last snapshot ID in this tree:
                 */
                struct bkey_i_snapshot_tree *s_t;

                BUG_ON(s.v->children[1]);

                s_t = bch2_bkey_get_mut_typed(trans, &tree_iter,
                                BTREE_ID_snapshot_trees, POS(0, le32_to_cpu(s.v->tree)),
                                0, snapshot_tree);
                ret = PTR_ERR_OR_ZERO(s_t);
                if (ret)
                        goto err;

                if (s.v->children[0]) {
                        s_t->v.root_snapshot = s.v->children[0];
                } else {
                        s_t->k.type = KEY_TYPE_deleted;
                        set_bkey_val_u64s(&s_t->k, 0);
                }
        }

        ret = bch2_btree_delete_at(trans, &iter, 0);
err:
        bch2_trans_iter_exit(trans, &tree_iter);
        bch2_trans_iter_exit(trans, &p_iter);
        bch2_trans_iter_exit(trans, &c_iter);
        bch2_trans_iter_exit(trans, &iter);
        return ret;
}

static int create_snapids(struct btree_trans *trans, u32 parent, u32 tree,
                          u32 *new_snapids,
                          u32 *snapshot_subvols,
                          unsigned nr_snapids)
{
        struct bch_fs *c = trans->c;
        struct btree_iter iter;
        struct bkey_i_snapshot *n;
        struct bkey_s_c k;
        unsigned i, j;
        u32 depth = bch2_snapshot_depth(c, parent);
        int ret;

        bch2_trans_iter_init(trans, &iter, BTREE_ID_snapshots,
                             POS_MIN, BTREE_ITER_INTENT);
        k = bch2_btree_iter_peek(&iter);
        ret = bkey_err(k);
        if (ret)
                goto err;

        for (i = 0; i < nr_snapids; i++) {
                k = bch2_btree_iter_prev_slot(&iter);
                ret = bkey_err(k);
                if (ret)
                        goto err;

                if (!k.k || !k.k->p.offset) {
                        ret = -BCH_ERR_ENOSPC_snapshot_create;
                        goto err;
                }

                n = bch2_bkey_alloc(trans, &iter, 0, snapshot);
                ret = PTR_ERR_OR_ZERO(n);
                if (ret)
                        goto err;

                n->v.flags      = 0;
                n->v.parent     = cpu_to_le32(parent);
                n->v.subvol     = cpu_to_le32(snapshot_subvols[i]);
                n->v.tree       = cpu_to_le32(tree);
                n->v.depth      = cpu_to_le32(depth);
                n->v.btime.lo   = cpu_to_le64(bch2_current_time(c));
                n->v.btime.hi   = 0;

                for (j = 0; j < ARRAY_SIZE(n->v.skip); j++)
                        n->v.skip[j] = cpu_to_le32(bch2_snapshot_skiplist_get(c, parent));

                bubble_sort(n->v.skip, ARRAY_SIZE(n->v.skip), cmp_le32);
                SET_BCH_SNAPSHOT_SUBVOL(&n->v, true);

                ret = __bch2_mark_snapshot(trans, BTREE_ID_snapshots, 0,
                                         bkey_s_c_null, bkey_i_to_s_c(&n->k_i), 0);
                if (ret)
                        goto err;

                new_snapids[i]  = iter.pos.offset;

                mutex_lock(&c->snapshot_table_lock);
                snapshot_t_mut(c, new_snapids[i])->equiv = new_snapids[i];
                mutex_unlock(&c->snapshot_table_lock);
        }
err:
        bch2_trans_iter_exit(trans, &iter);
        return ret;
}

/*
 * Create new snapshot IDs as children of an existing snapshot ID:
 */
static int bch2_snapshot_node_create_children(struct btree_trans *trans, u32 parent,
                              u32 *new_snapids,
                              u32 *snapshot_subvols,
                              unsigned nr_snapids)
{
        struct btree_iter iter;
        struct bkey_i_snapshot *n_parent;
        int ret = 0;

        n_parent = bch2_bkey_get_mut_typed(trans, &iter,
                        BTREE_ID_snapshots, POS(0, parent),
                        0, snapshot);
        ret = PTR_ERR_OR_ZERO(n_parent);
        if (unlikely(ret)) {
                if (bch2_err_matches(ret, ENOENT))
                        bch_err(trans->c, "snapshot %u not found", parent);
                return ret;
        }

        if (n_parent->v.children[0] || n_parent->v.children[1]) {
                bch_err(trans->c, "Trying to add child snapshot nodes to parent that already has children");
                ret = -EINVAL;
                goto err;
        }

        ret = create_snapids(trans, parent, le32_to_cpu(n_parent->v.tree),
                             new_snapids, snapshot_subvols, nr_snapids);
        if (ret)
                goto err;

        n_parent->v.children[0] = cpu_to_le32(new_snapids[0]);
        n_parent->v.children[1] = cpu_to_le32(new_snapids[1]);
        n_parent->v.subvol = 0;
        SET_BCH_SNAPSHOT_SUBVOL(&n_parent->v, false);
err:
        bch2_trans_iter_exit(trans, &iter);
        return ret;
}

/*
 * Create a snapshot node that is the root of a new tree:
 */
static int bch2_snapshot_node_create_tree(struct btree_trans *trans,
                              u32 *new_snapids,
                              u32 *snapshot_subvols,
                              unsigned nr_snapids)
{
        struct bkey_i_snapshot_tree *n_tree;
        int ret;

        n_tree = __bch2_snapshot_tree_create(trans);
        ret =   PTR_ERR_OR_ZERO(n_tree) ?:
                create_snapids(trans, 0, n_tree->k.p.offset,
                             new_snapids, snapshot_subvols, nr_snapids);
        if (ret)
                return ret;

        n_tree->v.master_subvol = cpu_to_le32(snapshot_subvols[0]);
        n_tree->v.root_snapshot = cpu_to_le32(new_snapids[0]);
        return 0;
}

int bch2_snapshot_node_create(struct btree_trans *trans, u32 parent,
                              u32 *new_snapids,
                              u32 *snapshot_subvols,
                              unsigned nr_snapids)
{
        BUG_ON((parent == 0) != (nr_snapids == 1));
        BUG_ON((parent != 0) != (nr_snapids == 2));

        return parent
                ? bch2_snapshot_node_create_children(trans, parent,
                                new_snapids, snapshot_subvols, nr_snapids)
                : bch2_snapshot_node_create_tree(trans,
                                new_snapids, snapshot_subvols, nr_snapids);

}

/*
 * If we have an unlinked inode in an internal snapshot node, and the inode
 * really has been deleted in all child snapshots, how does this get cleaned up?
 *
 * first there is the problem of how keys that have been overwritten in all
 * child snapshots get deleted (unimplemented?), but inodes may perhaps be
 * special?
 *
 * also: unlinked inode in internal snapshot appears to not be getting deleted
 * correctly if inode doesn't exist in leaf snapshots
 *
 * solution:
 *
 * for a key in an interior snapshot node that needs work to be done that
 * requires it to be mutated: iterate over all descendent leaf nodes and copy
 * that key to snapshot leaf nodes, where we can mutate it
 */

static int snapshot_delete_key(struct btree_trans *trans,
                               struct btree_iter *iter,
                               struct bkey_s_c k,
                               snapshot_id_list *deleted,
                               snapshot_id_list *equiv_seen,
                               struct bpos *last_pos)
{
        struct bch_fs *c = trans->c;
        u32 equiv = bch2_snapshot_equiv(c, k.k->p.snapshot);

        if (!bkey_eq(k.k->p, *last_pos))
                equiv_seen->nr = 0;
        *last_pos = k.k->p;

        if (snapshot_list_has_id(deleted, k.k->p.snapshot) ||
            snapshot_list_has_id(equiv_seen, equiv)) {
                return bch2_btree_delete_at(trans, iter,
                                            BTREE_UPDATE_INTERNAL_SNAPSHOT_NODE);
        } else {
                return snapshot_list_add(c, equiv_seen, equiv);
        }
}

static int move_key_to_correct_snapshot(struct btree_trans *trans,
                               struct btree_iter *iter,
                               struct bkey_s_c k)
{
        struct bch_fs *c = trans->c;
        u32 equiv = bch2_snapshot_equiv(c, k.k->p.snapshot);

        /*
         * When we have a linear chain of snapshot nodes, we consider
         * those to form an equivalence class: we're going to collapse
         * them all down to a single node, and keep the leaf-most node -
         * which has the same id as the equivalence class id.
         *
         * If there are multiple keys in different snapshots at the same
         * position, we're only going to keep the one in the newest
         * snapshot - the rest have been overwritten and are redundant,
         * and for the key we're going to keep we need to move it to the
         * equivalance class ID if it's not there already.
         */
        if (equiv != k.k->p.snapshot) {
                struct bkey_i *new = bch2_bkey_make_mut_noupdate(trans, k);
                struct btree_iter new_iter;
                int ret;

                ret = PTR_ERR_OR_ZERO(new);
                if (ret)
                        return ret;

                new->k.p.snapshot = equiv;

                bch2_trans_iter_init(trans, &new_iter, iter->btree_id, new->k.p,
                                     BTREE_ITER_ALL_SNAPSHOTS|
                                     BTREE_ITER_CACHED|
                                     BTREE_ITER_INTENT);

                ret =   bch2_btree_iter_traverse(&new_iter) ?:
                        bch2_trans_update(trans, &new_iter, new,
                                        BTREE_UPDATE_INTERNAL_SNAPSHOT_NODE) ?:
                        bch2_btree_delete_at(trans, iter,
                                        BTREE_UPDATE_INTERNAL_SNAPSHOT_NODE);
                bch2_trans_iter_exit(trans, &new_iter);
                if (ret)
                        return ret;
        }

        return 0;
}

static int bch2_snapshot_needs_delete(struct btree_trans *trans, struct bkey_s_c k)
{
        struct bkey_s_c_snapshot snap;
        u32 children[2];
        int ret;

        if (k.k->type != KEY_TYPE_snapshot)
                return 0;

        snap = bkey_s_c_to_snapshot(k);
        if (BCH_SNAPSHOT_DELETED(snap.v) ||
            BCH_SNAPSHOT_SUBVOL(snap.v))
                return 0;

        children[0] = le32_to_cpu(snap.v->children[0]);
        children[1] = le32_to_cpu(snap.v->children[1]);

        ret   = bch2_snapshot_live(trans, children[0]) ?:
                bch2_snapshot_live(trans, children[1]);
        if (ret < 0)
                return ret;
        return !ret;
}

/*
 * For a given snapshot, if it doesn't have a subvolume that points to it, and
 * it doesn't have child snapshot nodes - it's now redundant and we can mark it
 * as deleted.
 */
static int bch2_delete_redundant_snapshot(struct btree_trans *trans, struct bkey_s_c k)
{
        int ret = bch2_snapshot_needs_delete(trans, k);

        return ret <= 0
                ? ret
                : bch2_snapshot_node_set_deleted(trans, k.k->p.offset);
}

static inline u32 bch2_snapshot_nth_parent_skip(struct bch_fs *c, u32 id, u32 n,
                                                snapshot_id_list *skip)
{
        rcu_read_lock();
        while (snapshot_list_has_id(skip, id))
                id = __bch2_snapshot_parent(c, id);

        while (n--) {
                do {
                        id = __bch2_snapshot_parent(c, id);
                } while (snapshot_list_has_id(skip, id));
        }
        rcu_read_unlock();

        return id;
}

static int bch2_fix_child_of_deleted_snapshot(struct btree_trans *trans,
                                              struct btree_iter *iter, struct bkey_s_c k,
                                              snapshot_id_list *deleted)
{
        struct bch_fs *c = trans->c;
        u32 nr_deleted_ancestors = 0;
        struct bkey_i_snapshot *s;
        int ret;

        if (k.k->type != KEY_TYPE_snapshot)
                return 0;

        if (snapshot_list_has_id(deleted, k.k->p.offset))
                return 0;

        s = bch2_bkey_make_mut_noupdate_typed(trans, k, snapshot);
        ret = PTR_ERR_OR_ZERO(s);
        if (ret)
                return ret;

        darray_for_each(*deleted, i)
                nr_deleted_ancestors += bch2_snapshot_is_ancestor(c, s->k.p.offset, *i);

        if (!nr_deleted_ancestors)
                return 0;

        le32_add_cpu(&s->v.depth, -nr_deleted_ancestors);

        if (!s->v.depth) {
                s->v.skip[0] = 0;
                s->v.skip[1] = 0;
                s->v.skip[2] = 0;
        } else {
                u32 depth = le32_to_cpu(s->v.depth);
                u32 parent = bch2_snapshot_parent(c, s->k.p.offset);

                for (unsigned j = 0; j < ARRAY_SIZE(s->v.skip); j++) {
                        u32 id = le32_to_cpu(s->v.skip[j]);

                        if (snapshot_list_has_id(deleted, id)) {
                                id = bch2_snapshot_nth_parent_skip(c,
                                                        parent,
                                                        depth > 1
                                                        ? get_random_u32_below(depth - 1)
                                                        : 0,
                                                        deleted);
                                s->v.skip[j] = cpu_to_le32(id);
                        }
                }

                bubble_sort(s->v.skip, ARRAY_SIZE(s->v.skip), cmp_le32);
        }

        return bch2_trans_update(trans, iter, &s->k_i, 0);
}

int bch2_delete_dead_snapshots(struct bch_fs *c)
{
        struct btree_trans *trans;
        snapshot_id_list deleted = { 0 };
        snapshot_id_list deleted_interior = { 0 };
        u32 id;
        int ret = 0;

        if (!test_and_clear_bit(BCH_FS_need_delete_dead_snapshots, &c->flags))
                return 0;

        if (!test_bit(BCH_FS_started, &c->flags)) {
                ret = bch2_fs_read_write_early(c);
                bch_err_msg(c, ret, "deleting dead snapshots: error going rw");
                if (ret)
                        return ret;
        }

        trans = bch2_trans_get(c);

        /*
         * For every snapshot node: If we have no live children and it's not
         * pointed to by a subvolume, delete it:
         */
        ret = for_each_btree_key_commit(trans, iter, BTREE_ID_snapshots,
                        POS_MIN, 0, k,
                        NULL, NULL, 0,
                bch2_delete_redundant_snapshot(trans, k));
        bch_err_msg(c, ret, "deleting redundant snapshots");
        if (ret)
                goto err;

        ret = for_each_btree_key(trans, iter, BTREE_ID_snapshots,
                                 POS_MIN, 0, k,
                bch2_snapshot_set_equiv(trans, k));
        bch_err_msg(c, ret, "in bch2_snapshots_set_equiv");
        if (ret)
                goto err;

        ret = for_each_btree_key(trans, iter, BTREE_ID_snapshots,
                                 POS_MIN, 0, k, ({
                if (k.k->type != KEY_TYPE_snapshot)
                        continue;

                BCH_SNAPSHOT_DELETED(bkey_s_c_to_snapshot(k).v)
                        ? snapshot_list_add(c, &deleted, k.k->p.offset)
                        : 0;
        }));
        bch_err_msg(c, ret, "walking snapshots");
        if (ret)
                goto err;

        for (id = 0; id < BTREE_ID_NR; id++) {
                struct bpos last_pos = POS_MIN;
                snapshot_id_list equiv_seen = { 0 };
                struct disk_reservation res = { 0 };

                if (!btree_type_has_snapshots(id))
                        continue;

                /*
                 * deleted inodes btree is maintained by a trigger on the inodes
                 * btree - no work for us to do here, and it's not safe to scan
                 * it because we'll see out of date keys due to the btree write
                 * buffer:
                 */
                if (id == BTREE_ID_deleted_inodes)
                        continue;

                ret = for_each_btree_key_commit(trans, iter,
                                id, POS_MIN,
                                BTREE_ITER_PREFETCH|BTREE_ITER_ALL_SNAPSHOTS, k,
                                &res, NULL, BCH_TRANS_COMMIT_no_enospc,
                        snapshot_delete_key(trans, &iter, k, &deleted, &equiv_seen, &last_pos)) ?:
                      for_each_btree_key_commit(trans, iter,
                                id, POS_MIN,
                                BTREE_ITER_PREFETCH|BTREE_ITER_ALL_SNAPSHOTS, k,
                                &res, NULL, BCH_TRANS_COMMIT_no_enospc,
                        move_key_to_correct_snapshot(trans, &iter, k));

                bch2_disk_reservation_put(c, &res);
                darray_exit(&equiv_seen);

                bch_err_msg(c, ret, "deleting keys from dying snapshots");
                if (ret)
                        goto err;
        }

        bch2_trans_unlock(trans);
        down_write(&c->snapshot_create_lock);

        ret = for_each_btree_key(trans, iter, BTREE_ID_snapshots,
                                 POS_MIN, 0, k, ({
                u32 snapshot = k.k->p.offset;
                u32 equiv = bch2_snapshot_equiv(c, snapshot);

                equiv != snapshot
                        ? snapshot_list_add(c, &deleted_interior, snapshot)
                        : 0;
        }));

        bch_err_msg(c, ret, "walking snapshots");
        if (ret)
                goto err_create_lock;

        /*
         * Fixing children of deleted snapshots can't be done completely
         * atomically, if we crash between here and when we delete the interior
         * nodes some depth fields will be off:
         */
        ret = for_each_btree_key_commit(trans, iter, BTREE_ID_snapshots, POS_MIN,
                                  BTREE_ITER_INTENT, k,
                                  NULL, NULL, BCH_TRANS_COMMIT_no_enospc,
                bch2_fix_child_of_deleted_snapshot(trans, &iter, k, &deleted_interior));
        if (ret)
                goto err_create_lock;

        darray_for_each(deleted, i) {
                ret = commit_do(trans, NULL, NULL, 0,
                        bch2_snapshot_node_delete(trans, *i));
                bch_err_msg(c, ret, "deleting snapshot %u", *i);
                if (ret)
                        goto err_create_lock;
        }

        darray_for_each(deleted_interior, i) {
                ret = commit_do(trans, NULL, NULL, 0,
                        bch2_snapshot_node_delete(trans, *i));
                bch_err_msg(c, ret, "deleting snapshot %u", *i);
                if (ret)
                        goto err_create_lock;
        }
err_create_lock:
        up_write(&c->snapshot_create_lock);
err:
        darray_exit(&deleted_interior);
        darray_exit(&deleted);
        bch2_trans_put(trans);
        bch_err_fn(c, ret);
        return ret;
}

void bch2_delete_dead_snapshots_work(struct work_struct *work)
{
        struct bch_fs *c = container_of(work, struct bch_fs, snapshot_delete_work);

        bch2_delete_dead_snapshots(c);
        bch2_write_ref_put(c, BCH_WRITE_REF_delete_dead_snapshots);
}

void bch2_delete_dead_snapshots_async(struct bch_fs *c)
{
        if (bch2_write_ref_tryget(c, BCH_WRITE_REF_delete_dead_snapshots) &&
            !queue_work(c->write_ref_wq, &c->snapshot_delete_work))
                bch2_write_ref_put(c, BCH_WRITE_REF_delete_dead_snapshots);
}

int __bch2_key_has_snapshot_overwrites(struct btree_trans *trans,
                                       enum btree_id id,
                                       struct bpos pos)
{
        struct bch_fs *c = trans->c;
        struct btree_iter iter;
        struct bkey_s_c k;
        int ret;

        bch2_trans_iter_init(trans, &iter, id, pos,
                             BTREE_ITER_NOT_EXTENTS|
                             BTREE_ITER_ALL_SNAPSHOTS);
        while (1) {
                k = bch2_btree_iter_prev(&iter);
                ret = bkey_err(k);
                if (ret)
                        break;

                if (!k.k)
                        break;

                if (!bkey_eq(pos, k.k->p))
                        break;

                if (bch2_snapshot_is_ancestor(c, k.k->p.snapshot, pos.snapshot)) {
                        ret = 1;
                        break;
                }
        }
        bch2_trans_iter_exit(trans, &iter);

        return ret;
}

static u32 bch2_snapshot_smallest_child(struct bch_fs *c, u32 id)
{
        const struct snapshot_t *s = snapshot_t(c, id);

        return s->children[1] ?: s->children[0];
}

static u32 bch2_snapshot_smallest_descendent(struct bch_fs *c, u32 id)
{
        u32 child;

        while ((child = bch2_snapshot_smallest_child(c, id)))
                id = child;
        return id;
}

static int bch2_propagate_key_to_snapshot_leaf(struct btree_trans *trans,
                                               enum btree_id btree,
                                               struct bkey_s_c interior_k,
                                               u32 leaf_id, struct bpos *new_min_pos)
{
        struct btree_iter iter;
        struct bpos pos = interior_k.k->p;
        struct bkey_s_c k;
        struct bkey_i *new;
        int ret;

        pos.snapshot = leaf_id;

        bch2_trans_iter_init(trans, &iter, btree, pos, BTREE_ITER_INTENT);
        k = bch2_btree_iter_peek_slot(&iter);
        ret = bkey_err(k);
        if (ret)
                goto out;

        /* key already overwritten in this snapshot? */
        if (k.k->p.snapshot != interior_k.k->p.snapshot)
                goto out;

        if (bpos_eq(*new_min_pos, POS_MIN)) {
                *new_min_pos = k.k->p;
                new_min_pos->snapshot = leaf_id;
        }

        new = bch2_bkey_make_mut_noupdate(trans, interior_k);
        ret = PTR_ERR_OR_ZERO(new);
        if (ret)
                goto out;

        new->k.p.snapshot = leaf_id;
        ret = bch2_trans_update(trans, &iter, new, 0);
out:
        bch2_trans_iter_exit(trans, &iter);
        return ret;
}

int bch2_propagate_key_to_snapshot_leaves(struct btree_trans *trans,
                                          enum btree_id btree,
                                          struct bkey_s_c k,
                                          struct bpos *new_min_pos)
{
        struct bch_fs *c = trans->c;
        struct bkey_buf sk;
        u32 restart_count = trans->restart_count;
        int ret = 0;

        bch2_bkey_buf_init(&sk);
        bch2_bkey_buf_reassemble(&sk, c, k);
        k = bkey_i_to_s_c(sk.k);

        *new_min_pos = POS_MIN;

        for (u32 id = bch2_snapshot_smallest_descendent(c, k.k->p.snapshot);
             id < k.k->p.snapshot;
             id++) {
                if (!bch2_snapshot_is_ancestor(c, id, k.k->p.snapshot) ||
                    !bch2_snapshot_is_leaf(c, id))
                        continue;
again:
                ret =   btree_trans_too_many_iters(trans) ?:
                        bch2_propagate_key_to_snapshot_leaf(trans, btree, k, id, new_min_pos) ?:
                        bch2_trans_commit(trans, NULL, NULL, 0);
                if (ret && bch2_err_matches(ret, BCH_ERR_transaction_restart)) {
                        bch2_trans_begin(trans);
                        goto again;
                }

                if (ret)
                        break;
        }

        bch2_bkey_buf_exit(&sk, c);

        return ret ?: trans_was_restarted(trans, restart_count);
}

static int bch2_check_snapshot_needs_deletion(struct btree_trans *trans, struct bkey_s_c k)
{
        struct bch_fs *c = trans->c;
        struct bkey_s_c_snapshot snap;
        int ret = 0;

        if (k.k->type != KEY_TYPE_snapshot)
                return 0;

        snap = bkey_s_c_to_snapshot(k);
        if (BCH_SNAPSHOT_DELETED(snap.v) ||
            bch2_snapshot_equiv(c, k.k->p.offset) != k.k->p.offset ||
            (ret = bch2_snapshot_needs_delete(trans, k)) > 0) {
                set_bit(BCH_FS_need_delete_dead_snapshots, &c->flags);
                return 0;
        }

        return ret;
}

int bch2_snapshots_read(struct bch_fs *c)
{
        int ret = bch2_trans_run(c,
                for_each_btree_key(trans, iter, BTREE_ID_snapshots,
                                   POS_MIN, 0, k,
                        __bch2_mark_snapshot(trans, BTREE_ID_snapshots, 0, bkey_s_c_null, k, 0) ?:
                        bch2_snapshot_set_equiv(trans, k) ?:
                        bch2_check_snapshot_needs_deletion(trans, k)) ?:
                for_each_btree_key(trans, iter, BTREE_ID_snapshots,
                                   POS_MIN, 0, k,
                           (set_is_ancestor_bitmap(c, k.k->p.offset), 0)));
        bch_err_fn(c, ret);
        return ret;
}

void bch2_fs_snapshots_exit(struct bch_fs *c)
{
        kvfree(rcu_dereference_protected(c->snapshots, true));
}