]> git.sesse.net Git - bcachefs-tools-debian/blobdiff - libbcachefs/subvolume.c
Update bcachefs sources to 33a60d9b05 bcachefs: Assorted fixes for clang
[bcachefs-tools-debian] / libbcachefs / subvolume.c
index 43d83705a7aee6ce209ca3c3023b058ac3eda1f0..736afb626a8945b75d4bc3300fb15fb60d594de8 100644 (file)
 #include "fs.h"
 #include "subvolume.h"
 
+#include <linux/random.h>
+
+static int bch2_subvolume_delete(struct btree_trans *, u32);
+
+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->curr_recovery_pass <= 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);
+
+       ret = id && id < ancestor
+               ? test_bit(ancestor - id - 1, __snapshot_t(t, id)->is_ancestor)
+               : id == ancestor;
+       rcu_read_unlock();
+
+       return ret;
+}
+
+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 bch2_snapshot_depth(struct bch_fs *c, u32 parent)
+{
+       u32 depth;
+
+       rcu_read_lock();
+       depth = parent ? snapshot_t(c, parent)->depth + 1 : 0;
+       rcu_read_unlock();
+
+       return depth;
+}
+
+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;
+       if (old)
+               kvfree_rcu(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);
+}
+
 /* 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(const struct bch_fs *c, struct bkey_s_c k,
+                              enum bkey_invalid_flags flags,
+                              struct printbuf *err)
+{
+       if (bkey_gt(k.k->p, POS(0, U32_MAX)) ||
+           bkey_lt(k.k->p, POS(0, 1))) {
+               prt_printf(err, "bad pos");
+               return -BCH_ERR_invalid_bkey;
+       }
+
+       return 0;
+}
+
+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;
+}
+
+static struct bkey_i_snapshot_tree *
+__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 snapshot_tree_create(struct btree_trans *trans,
+                               u32 root_id, u32 subvol_id, u32 *tree_id)
+{
+       struct bkey_i_snapshot_tree *n_tree =
+               __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: */
+
 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",
+       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->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(const struct bch_fs *c, struct bkey_s_c k,
-                         unsigned flags, struct printbuf *err)
+                         enum bkey_invalid_flags flags,
+                         struct printbuf *err)
 {
        struct bkey_s_c_snapshot s;
        u32 i, id;
@@ -36,12 +216,6 @@ int bch2_snapshot_invalid(const struct bch_fs *c, struct bkey_s_c k,
                return -BCH_ERR_invalid_bkey;
        }
 
-       if (bkey_val_bytes(k.k) != sizeof(struct bch_snapshot)) {
-               prt_printf(err, "bad val size (%zu != %zu)",
-                      bkey_val_bytes(k.k), sizeof(struct bch_snapshot));
-               return -BCH_ERR_invalid_bkey;
-       }
-
        s = bkey_s_c_to_snapshot(k);
 
        id = le32_to_cpu(s.v->parent);
@@ -72,6 +246,25 @@ int bch2_snapshot_invalid(const struct bch_fs *c, struct bkey_s_c k,
                }
        }
 
+       if (bkey_val_bytes(k.k) > offsetof(struct bch_snapshot, skip)) {
+               if (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])) {
+                       prt_printf(err, "skiplist not normalized");
+                       return -BCH_ERR_invalid_bkey;
+               }
+
+               for (i = 0; i < ARRAY_SIZE(s.v->skip); i++) {
+                       id = le32_to_cpu(s.v->skip[i]);
+
+                       if (!id != !s.v->parent ||
+                           (s.v->parent &&
+                            id <= k.k->p.offset)) {
+                               prt_printf(err, "bad skiplist node %u)", id);
+                               return -BCH_ERR_invalid_bkey;
+                       }
+               }
+       }
+
        return 0;
 }
 
@@ -82,47 +275,60 @@ int bch2_mark_snapshot(struct btree_trans *trans,
 {
        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 = genradix_ptr_alloc(&c->snapshots,
-                              U32_MAX - new.k->p.offset,
-                              GFP_KERNEL);
-       if (!t)
-               return -BCH_ERR_ENOMEM_mark_snapshot;
+       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);
+               u32 parent = id;
 
                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;
+               }
+
+               while ((parent = bch2_snapshot_parent_early(c, parent)) &&
+                      parent - id - 1 < IS_ANCESTOR_BITMAP)
+                       __set_bit(parent - id - 1, t->is_ancestor);
+
+               if (BCH_SNAPSHOT_DELETED(s.v)) {
+                       set_bit(BCH_FS_HAVE_DELETED_SNAPSHOTS, &c->flags);
+                       c->recovery_passes_explicit |= BIT_ULL(BCH_RECOVERY_PASS_delete_dead_snapshots);
+               }
        } else {
-               t->parent       = 0;
-               t->children[0]  = 0;
-               t->children[1]  = 0;
-               t->subvol       = 0;
+               memset(t, 0, sizeof(*t));
        }
-
-       return 0;
+err:
+       mutex_unlock(&c->snapshot_table_lock);
+       return ret;
 }
 
 static int snapshot_lookup(struct btree_trans *trans, u32 id,
                           struct bch_snapshot *s)
 {
-       struct btree_iter iter;
-       struct bkey_s_c k;
-       int ret;
-
-       bch2_trans_iter_init(trans, &iter, BTREE_ID_snapshots, POS(0, id),
-                            BTREE_ITER_WITH_UPDATES);
-       k = bch2_btree_iter_peek_slot(&iter);
-       ret = bkey_err(k) ?: k.k->type == KEY_TYPE_snapshot ? 0 : -ENOENT;
-
-       if (!ret)
-               *s = *bkey_s_c_to_snapshot(k).v;
-
-       bch2_trans_iter_exit(trans, &iter);
-       return ret;
+       return bch2_bkey_get_val_typed(trans, BTREE_ID_snapshots, POS(0, id),
+                                      BTREE_ITER_WITH_UPDATES, snapshot, s);
 }
 
 static int snapshot_live(struct btree_trans *trans, u32 id)
@@ -134,7 +340,7 @@ static int snapshot_live(struct btree_trans *trans, u32 id)
                return 0;
 
        ret = snapshot_lookup(trans, id, &v);
-       if (ret == -ENOENT)
+       if (bch2_err_matches(ret, ENOENT))
                bch_err(trans->c, "snapshot node %u not found", id);
        if (ret)
                return ret;
@@ -168,21 +374,346 @@ static int bch2_snapshot_set_equiv(struct btree_trans *trans, struct bkey_s_c k)
                nr_live += ret;
        }
 
-       snapshot_t(c, id)->equiv = nr_live == 1
-               ? snapshot_t(c, child[live_idx])->equiv
+       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;
+       struct bkey_s_c_subvolume s;
+       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;
+
+               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 *s;
+
+               *subvol_id = bch2_snapshot_tree_oldest_subvol(c, snapshot_root);
+
+               s = bch2_bkey_get_mut_typed(trans, &iter,
+                                           BTREE_ID_subvolumes, POS(0, *subvol_id),
+                                           0, subvolume);
+               ret = PTR_ERR_OR_ZERO(s);
+               if (ret)
+                       return ret;
+
+               SET_BCH_SUBVOLUME_SNAP(&s->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 = 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 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 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 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 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)
+{
+       struct btree_iter iter;
+       struct bkey_s_c k;
+       int ret;
+
+       ret = bch2_trans_run(c,
+               for_each_btree_key_commit(&trans, iter,
+                       BTREE_ID_snapshot_trees, POS_MIN,
+                       BTREE_ITER_PREFETCH, k,
+                       NULL, NULL, BTREE_INSERT_LAZY_RW|BTREE_INSERT_NOFAIL,
+               check_snapshot_tree(&trans, &iter, k)));
+
+       if (ret)
+               bch_err(c, "error %i checking snapshot trees", 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));
+}
+
+static u32 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, struct bch_snapshot s)
+{
+       struct bch_snapshot a;
+       unsigned i;
+       int ret;
+
+       for (i = 0; i < 3; i++) {
+               if (!s.parent != !s.skip[i])
+                       return false;
+
+               if (!s.parent)
+                       continue;
+
+               ret = snapshot_lookup(trans, le32_to_cpu(s.skip[i]), &a);
+               if (bch2_err_matches(ret, ENOENT))
+                       return false;
+               if (ret)
+                       return ret;
+
+               if (a.tree != s.tree)
+                       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) ?:
+                       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 cmp_le32(__le32 l, __le32 r)
+{
+       return cmp_int(le32_to_cpu(l), le32_to_cpu(r));
+}
+
 static int check_snapshot(struct btree_trans *trans,
                          struct btree_iter *iter,
                          struct bkey_s_c k)
 {
        struct bch_fs *c = trans->c;
-       struct bkey_s_c_snapshot s;
+       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;
@@ -191,105 +722,147 @@ static int check_snapshot(struct btree_trans *trans,
        if (k.k->type != KEY_TYPE_snapshot)
                return 0;
 
-       s = bkey_s_c_to_snapshot(k);
-       id = le32_to_cpu(s.v->parent);
+       memset(&s, 0, sizeof(s));
+       memcpy(&s, k.v, bkey_val_bytes(k.k));
+
+       id = le32_to_cpu(s.parent);
        if (id) {
                ret = snapshot_lookup(trans, id, &v);
-               if (ret == -ENOENT)
+               if (bch2_err_matches(ret, ENOENT))
                        bch_err(c, "snapshot with nonexistent parent:\n  %s",
-                               (bch2_bkey_val_to_text(&buf, c, s.s_c), buf.buf));
+                               (bch2_bkey_val_to_text(&buf, c, k), buf.buf));
                if (ret)
                        goto err;
 
-               if (le32_to_cpu(v.children[0]) != s.k->p.offset &&
-                   le32_to_cpu(v.children[1]) != s.k->p.offset) {
+               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, s.k->p.offset);
+                               id, k.k->p.offset);
                        ret = -EINVAL;
                        goto err;
                }
        }
 
-       for (i = 0; i < 2 && s.v->children[i]; i++) {
-               id = le32_to_cpu(s.v->children[i]);
+       for (i = 0; i < 2 && s.children[i]; i++) {
+               id = le32_to_cpu(s.children[i]);
 
                ret = snapshot_lookup(trans, id, &v);
-               if (ret == -ENOENT)
+               if (bch2_err_matches(ret, ENOENT))
                        bch_err(c, "snapshot node %llu has nonexistent child %u",
-                               s.k->p.offset, id);
+                               k.k->p.offset, id);
                if (ret)
                        goto err;
 
-               if (le32_to_cpu(v.parent) != s.k->p.offset) {
+               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), s.k->p.offset);
+                               id, le32_to_cpu(v.parent), k.k->p.offset);
                        ret = -EINVAL;
                        goto err;
                }
        }
 
-       should_have_subvol = BCH_SNAPSHOT_SUBVOL(s.v) &&
-               !BCH_SNAPSHOT_DELETED(s.v);
+       should_have_subvol = BCH_SNAPSHOT_SUBVOL(&s) &&
+               !BCH_SNAPSHOT_DELETED(&s);
 
        if (should_have_subvol) {
-               id = le32_to_cpu(s.v->subvol);
+               id = le32_to_cpu(s.subvol);
                ret = bch2_subvolume_get(trans, id, 0, false, &subvol);
-               if (ret == -ENOENT)
+               if (bch2_err_matches(ret, ENOENT))
                        bch_err(c, "snapshot points to nonexistent subvolume:\n  %s",
-                               (bch2_bkey_val_to_text(&buf, c, s.s_c), buf.buf));
+                               (bch2_bkey_val_to_text(&buf, c, k), buf.buf));
                if (ret)
                        goto err;
 
-               if (BCH_SNAPSHOT_SUBVOL(s.v) != (le32_to_cpu(subvol.snapshot) == s.k->p.offset)) {
+               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",
-                               s.k->p.offset);
+                               k.k->p.offset);
                        ret = -EINVAL;
                        goto err;
                }
        } else {
-               if (fsck_err_on(s.v->subvol, c, "snapshot should not point to subvol:\n  %s",
-                               (bch2_bkey_val_to_text(&buf, c, s.s_c), buf.buf))) {
-                       struct bkey_i_snapshot *u = bch2_trans_kmalloc(trans, sizeof(*u));
-
+               if (fsck_err_on(s.subvol, c, "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;
 
-                       bkey_reassemble(&u->k_i, s.s_c);
                        u->v.subvol = 0;
-                       ret = bch2_trans_update(trans, iter, &u->k_i, 0);
-                       if (ret)
-                               goto err;
+                       s = u->v;
                }
        }
 
-       if (BCH_SNAPSHOT_DELETED(s.v))
-               set_bit(BCH_FS_HAVE_DELETED_SNAPSHOTS, &c->flags);
+       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 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 (le32_to_cpu(s.depth) != real_depth &&
+           (c->sb.version_upgrade_complete < bcachefs_metadata_version_snapshot_skiplists ||
+            fsck_err(c, "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, s);
+       if (ret < 0)
+               goto err;
+
+       if (!ret &&
+           (c->sb.version_upgrade_complete < bcachefs_metadata_version_snapshot_skiplists ||
+            fsck_err(c, "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(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_fs_check_snapshots(struct bch_fs *c)
+int bch2_check_snapshots(struct bch_fs *c)
 {
-       struct btree_trans trans;
        struct btree_iter iter;
        struct bkey_s_c k;
        int ret;
 
-       bch2_trans_init(&trans, c, 0, 0);
-
-       ret = for_each_btree_key_commit(&trans, iter,
-                       BTREE_ID_snapshots, POS_MIN,
+       /*
+        * We iterate backwards as checking/fixing the depth field requires that
+        * the parent's depth already be correct:
+        */
+       ret = bch2_trans_run(c,
+               for_each_btree_key_reverse_commit(&trans, iter,
+                       BTREE_ID_snapshots, POS_MAX,
                        BTREE_ITER_PREFETCH, k,
                        NULL, NULL, BTREE_INSERT_LAZY_RW|BTREE_INSERT_NOFAIL,
-               check_snapshot(&trans, &iter, k));
-
+               check_snapshot(&trans, &iter, k)));
        if (ret)
-               bch_err(c, "error %i checking snapshots", ret);
-
-       bch2_trans_exit(&trans);
+               bch_err_fn(c, ret);
        return ret;
 }
 
@@ -297,10 +870,11 @@ static int check_subvol(struct btree_trans *trans,
                        struct btree_iter *iter,
                        struct bkey_s_c k)
 {
+       struct bch_fs *c = trans->c;
        struct bkey_s_c_subvolume subvol;
        struct bch_snapshot snapshot;
        unsigned snapid;
-       int ret;
+       int ret = 0;
 
        if (k.k->type != KEY_TYPE_subvolume)
                return 0;
@@ -309,66 +883,90 @@ static int check_subvol(struct btree_trans *trans,
        snapid = le32_to_cpu(subvol.v->snapshot);
        ret = snapshot_lookup(trans, snapid, &snapshot);
 
-       if (ret == -ENOENT)
-               bch_err(trans->c, "subvolume %llu points to nonexistent snapshot %u",
+       if (bch2_err_matches(ret, ENOENT))
+               bch_err(c, "subvolume %llu points to nonexistent snapshot %u",
                        k.k->p.offset, snapid);
        if (ret)
                return ret;
 
        if (BCH_SUBVOLUME_UNLINKED(subvol.v)) {
+               bch2_fs_lazy_rw(c);
+
                ret = bch2_subvolume_delete(trans, iter->pos.offset);
-               if (ret && !bch2_err_matches(ret, BCH_ERR_transaction_restart))
-                       bch_err(trans->c, "error deleting subvolume %llu: %s",
+               if (ret)
+                       bch_err(c, "error deleting subvolume %llu: %s",
                                iter->pos.offset, bch2_err_str(ret));
+               return ret ?: -BCH_ERR_transaction_restart_nested;
+       }
+
+       if (!BCH_SUBVOLUME_SNAP(subvol.v)) {
+               u32 snapshot_root = bch2_snapshot_root(c, le32_to_cpu(subvol.v->snapshot));
+               u32 snapshot_tree;
+               struct bch_snapshot_tree st;
+
+               rcu_read_lock();
+               snapshot_tree = snapshot_t(c, snapshot_root)->tree;
+               rcu_read_unlock();
+
+               ret = bch2_snapshot_tree_lookup(trans, snapshot_tree, &st);
+
+               bch2_fs_inconsistent_on(bch2_err_matches(ret, ENOENT), c,
+                               "%s: snapshot tree %u not found", __func__, snapshot_tree);
+
                if (ret)
                        return ret;
+
+               if (fsck_err_on(le32_to_cpu(st.master_subvol) != subvol.k->p.offset, c,
+                               "subvolume %llu is not set as snapshot but is not master subvolume",
+                               k.k->p.offset)) {
+                       struct bkey_i_subvolume *s =
+                               bch2_bkey_make_mut_typed(trans, iter, &subvol.s_c, 0, subvolume);
+                       ret = PTR_ERR_OR_ZERO(s);
+                       if (ret)
+                               return ret;
+
+                       SET_BCH_SUBVOLUME_SNAP(&s->v, true);
+               }
        }
 
-       return 0;
+fsck_err:
+       return ret;
 }
 
-int bch2_fs_check_subvols(struct bch_fs *c)
+int bch2_check_subvols(struct bch_fs *c)
 {
-       struct btree_trans trans;
        struct btree_iter iter;
        struct bkey_s_c k;
        int ret;
 
-       bch2_trans_init(&trans, c, 0, 0);
-
-       ret = for_each_btree_key_commit(&trans, iter,
+       ret = bch2_trans_run(c,
+               for_each_btree_key_commit(&trans, iter,
                        BTREE_ID_subvolumes, POS_MIN, BTREE_ITER_PREFETCH, k,
                        NULL, NULL, BTREE_INSERT_LAZY_RW|BTREE_INSERT_NOFAIL,
-               check_subvol(&trans, &iter, k));
-
-       bch2_trans_exit(&trans);
-
+               check_subvol(&trans, &iter, k)));
+       if (ret)
+               bch_err_fn(c, ret);
        return ret;
 }
 
 void bch2_fs_snapshots_exit(struct bch_fs *c)
 {
-       genradix_free(&c->snapshots);
+       kfree(rcu_dereference_protected(c->snapshots, true));
 }
 
-int bch2_fs_snapshots_start(struct bch_fs *c)
+int bch2_snapshots_read(struct bch_fs *c)
 {
-       struct btree_trans trans;
        struct btree_iter iter;
        struct bkey_s_c k;
        int ret = 0;
 
-       bch2_trans_init(&trans, c, 0, 0);
-
-       for_each_btree_key2(&trans, iter, BTREE_ID_snapshots,
+       ret = bch2_trans_run(c,
+               for_each_btree_key2(&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_trans_exit(&trans);
-
+                       bch2_mark_snapshot(&trans, BTREE_ID_snapshots, 0, bkey_s_c_null, k, 0) ?:
+                       bch2_snapshot_set_equiv(&trans, k)));
        if (ret)
-               bch_err(c, "error starting snapshots: %s", bch2_err_str(ret));
+               bch_err_fn(c, ret);
        return ret;
 }
 
@@ -381,13 +979,14 @@ static int bch2_snapshot_node_set_deleted(struct btree_trans *trans, u32 id)
        struct bkey_i_snapshot *s;
        int ret = 0;
 
-       bch2_trans_iter_init(trans, &iter, BTREE_ID_snapshots, POS(0, id),
-                            BTREE_ITER_INTENT);
-       s = bch2_bkey_get_mut_typed(trans, &iter, snapshot);
+       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(ret == -ENOENT, trans->c, "missing snapshot %u", id);
-               goto err;
+               bch2_fs_inconsistent_on(bch2_err_matches(ret, ENOENT),
+                                       trans->c, "missing snapshot %u", id);
+               return ret;
        }
 
        /* already deleted? */
@@ -397,10 +996,6 @@ static int bch2_snapshot_node_set_deleted(struct btree_trans *trans, u32 id)
        SET_BCH_SNAPSHOT_DELETED(&s->v, true);
        SET_BCH_SNAPSHOT_SUBVOL(&s->v, false);
        s->v.subvol = 0;
-
-       ret = bch2_trans_update(trans, &iter, &s->k_i, 0);
-       if (ret)
-               goto err;
 err:
        bch2_trans_iter_exit(trans, &iter);
        return ret;
@@ -408,27 +1003,22 @@ err:
 
 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 bkey_s_c k;
+       struct btree_iter tree_iter = (struct btree_iter) { NULL };
        struct bkey_s_c_snapshot s;
        u32 parent_id;
        unsigned i;
        int ret = 0;
 
-       bch2_trans_iter_init(trans, &iter, BTREE_ID_snapshots, POS(0, id),
-                            BTREE_ITER_INTENT);
-       k = bch2_btree_iter_peek_slot(&iter);
-       ret = bkey_err(k);
-       if (ret)
-               goto err;
+       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 (k.k->type != KEY_TYPE_snapshot) {
-               bch2_fs_inconsistent(trans->c, "missing snapshot %u", id);
-               ret = -ENOENT;
+       if (ret)
                goto err;
-       }
-
-       s = bkey_s_c_to_snapshot(k);
 
        BUG_ON(!BCH_SNAPSHOT_DELETED(s.v));
        parent_id = le32_to_cpu(s.v->parent);
@@ -436,13 +1026,13 @@ static int bch2_snapshot_node_delete(struct btree_trans *trans, u32 id)
        if (parent_id) {
                struct bkey_i_snapshot *parent;
 
-               bch2_trans_iter_init(trans, &p_iter, BTREE_ID_snapshots,
-                                    POS(0, parent_id),
-                                    BTREE_ITER_INTENT);
-               parent = bch2_bkey_get_mut_typed(trans, &p_iter, snapshot);
+               parent = bch2_bkey_get_mut_typed(trans, &p_iter,
+                                    BTREE_ID_snapshots, POS(0, parent_id),
+                                    0, snapshot);
                ret = PTR_ERR_OR_ZERO(parent);
                if (unlikely(ret)) {
-                       bch2_fs_inconsistent_on(ret == -ENOENT, trans->c, "missing snapshot %u", parent_id);
+                       bch2_fs_inconsistent_on(bch2_err_matches(ret, ENOENT), c,
+                                               "missing snapshot %u", parent_id);
                        goto err;
                }
 
@@ -451,7 +1041,7 @@ static int bch2_snapshot_node_delete(struct btree_trans *trans, u32 id)
                                break;
 
                if (i == 2)
-                       bch_err(trans->c, "snapshot %u missing child pointer to %u",
+                       bch_err(c, "snapshot %u missing child pointer to %u",
                                parent_id, id);
                else
                        parent->v.children[i] = 0;
@@ -460,29 +1050,51 @@ static int bch2_snapshot_node_delete(struct btree_trans *trans, u32 id)
                    le32_to_cpu(parent->v.children[1]))
                        swap(parent->v.children[0],
                             parent->v.children[1]);
+       } else {
+               /*
+                * 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;
 
-               ret = bch2_trans_update(trans, &p_iter, &parent->k_i, 0);
+               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, &iter);
        return ret;
 }
 
-int bch2_snapshot_node_create(struct btree_trans *trans, u32 parent,
-                             u32 *new_snapids,
-                             u32 *snapshot_subvols,
-                             unsigned nr_snapids)
+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;
-       int ret = 0;
+       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);
@@ -502,7 +1114,7 @@ int bch2_snapshot_node_create(struct btree_trans *trans, u32 parent,
                        goto err;
                }
 
-               n = bch2_bkey_alloc(trans, &iter, snapshot);
+               n = bch2_bkey_alloc(trans, &iter, 0, snapshot);
                ret = PTR_ERR_OR_ZERO(n);
                if (ret)
                        goto err;
@@ -510,47 +1122,108 @@ int bch2_snapshot_node_create(struct btree_trans *trans, u32 parent,
                n->v.flags      = 0;
                n->v.parent     = cpu_to_le32(parent);
                n->v.subvol     = cpu_to_le32(snapshot_subvols[i]);
-               n->v.pad        = 0;
+               n->v.tree       = cpu_to_le32(tree);
+               n->v.depth      = cpu_to_le32(depth);
+
+               for (j = 0; j < ARRAY_SIZE(n->v.skip); j++)
+                       n->v.skip[j] = cpu_to_le32(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_trans_update(trans, &iter, &n->k_i, 0) ?:
-                       bch2_mark_snapshot(trans, BTREE_ID_snapshots, 0,
-                                          bkey_s_c_null, bkey_i_to_s_c(&n->k_i), 0);
+               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;
        }
+err:
+       bch2_trans_iter_exit(trans, &iter);
+       return ret;
+}
 
-       if (parent) {
-               bch2_btree_iter_set_pos(&iter, POS(0, parent));
-               n = bch2_bkey_get_mut_typed(trans, &iter, snapshot);
-               ret = PTR_ERR_OR_ZERO(n);
-               if (unlikely(ret)) {
-                       if (ret == -ENOENT)
-                               bch_err(trans->c, "snapshot %u not found", parent);
-                       goto err;
-               }
+/*
+ * 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;
 
-               if (n->v.children[0] || n->v.children[1]) {
-                       bch_err(trans->c, "Trying to add child snapshot nodes to parent that already has children");
-                       ret = -EINVAL;
-                       goto err;
-               }
+       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;
+       }
 
-               n->v.children[0] = cpu_to_le32(new_snapids[0]);
-               n->v.children[1] = cpu_to_le32(new_snapids[1]);
-               n->v.subvol = 0;
-               SET_BCH_SNAPSHOT_SUBVOL(&n->v, false);
-               ret   = bch2_trans_update(trans, &iter, &n->k_i, 0);
-               if (ret)
-                       goto err;
+       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 = __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);
+
+}
+
 static int snapshot_delete_key(struct btree_trans *trans,
                               struct btree_iter *iter,
                               struct bkey_s_c k,
@@ -559,7 +1232,7 @@ static int snapshot_delete_key(struct btree_trans *trans,
                               struct bpos *last_pos)
 {
        struct bch_fs *c = trans->c;
-       u32 equiv = snapshot_t(c, k.k->p.snapshot)->equiv;
+       u32 equiv = bch2_snapshot_equiv(c, k.k->p.snapshot);
 
        if (!bkey_eq(k.k->p, *last_pos))
                equiv_seen->nr = 0;
@@ -612,9 +1285,6 @@ int bch2_delete_dead_snapshots(struct bch_fs *c)
        u32 i, id;
        int ret = 0;
 
-       if (!test_bit(BCH_FS_HAVE_DELETED_SNAPSHOTS, &c->flags))
-               return 0;
-
        if (!test_bit(BCH_FS_STARTED, &c->flags)) {
                ret = bch2_fs_read_write_early(c);
                if (ret) {
@@ -700,6 +1370,8 @@ int bch2_delete_dead_snapshots(struct bch_fs *c)
 err:
        darray_exit(&deleted);
        bch2_trans_exit(&trans);
+       if (ret)
+               bch_err_fn(c, ret);
        return ret;
 }
 
@@ -707,14 +1379,15 @@ static 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);
+       if (test_bit(BCH_FS_HAVE_DELETED_SNAPSHOTS, &c->flags))
+               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(system_long_wq, &c->snapshot_delete_work))
+           !queue_work(c->write_ref_wq, &c->snapshot_delete_work))
                bch2_write_ref_put(c, BCH_WRITE_REF_delete_dead_snapshots);
 }
 
@@ -725,7 +1398,7 @@ static int bch2_delete_dead_snapshots_hook(struct btree_trans *trans,
 
        set_bit(BCH_FS_HAVE_DELETED_SNAPSHOTS, &c->flags);
 
-       if (!test_bit(BCH_FS_FSCK_DONE, &c->flags))
+       if (c->curr_recovery_pass <= BCH_RECOVERY_PASS_delete_dead_snapshots)
                return 0;
 
        bch2_delete_dead_snapshots_async(c);
@@ -743,12 +1416,6 @@ int bch2_subvolume_invalid(const struct bch_fs *c, struct bkey_s_c k,
                return -BCH_ERR_invalid_bkey;
        }
 
-       if (bkey_val_bytes(k.k) != sizeof(struct bch_subvolume)) {
-               prt_printf(err, "incorrect value size (%zu != %zu)",
-                      bkey_val_bytes(k.k), sizeof(struct bch_subvolume));
-               return -BCH_ERR_invalid_bkey;
-       }
-
        return 0;
 }
 
@@ -758,8 +1425,11 @@ void bch2_subvolume_to_text(struct printbuf *out, struct bch_fs *c,
        struct bkey_s_c_subvolume s = bkey_s_c_to_subvolume(k);
 
        prt_printf(out, "root %llu snapshot id %u",
-              le64_to_cpu(s.v->inode),
-              le32_to_cpu(s.v->snapshot));
+                  le64_to_cpu(s.v->inode),
+                  le32_to_cpu(s.v->snapshot));
+
+       if (bkey_val_bytes(s.k) > offsetof(struct bch_subvolume, parent))
+               prt_printf(out, " parent %u", le32_to_cpu(s.v->parent));
 }
 
 static __always_inline int
@@ -768,21 +1438,11 @@ bch2_subvolume_get_inlined(struct btree_trans *trans, unsigned subvol,
                           int iter_flags,
                           struct bch_subvolume *s)
 {
-       struct btree_iter iter;
-       struct bkey_s_c k;
-       int ret;
-
-       bch2_trans_iter_init(trans, &iter, BTREE_ID_subvolumes, POS(0, subvol),
-                            iter_flags);
-       k = bch2_btree_iter_peek_slot(&iter);
-       ret = bkey_err(k) ?: k.k->type == KEY_TYPE_subvolume ? 0 : -ENOENT;
-
-       if (ret == -ENOENT && inconsistent_if_not_found)
-               bch2_fs_inconsistent(trans->c, "missing subvolume %u", subvol);
-       if (!ret)
-               *s = *bkey_s_c_to_subvolume(k).v;
-
-       bch2_trans_iter_exit(trans, &iter);
+       int ret = bch2_bkey_get_val_typed(trans, BTREE_ID_subvolumes, POS(0, subvol),
+                                         iter_flags, subvolume, s);
+       bch2_fs_inconsistent_on(bch2_err_matches(ret, ENOENT) &&
+                               inconsistent_if_not_found,
+                               trans->c, "missing subvolume %u", subvol);
        return ret;
 }
 
@@ -806,47 +1466,88 @@ int bch2_snapshot_get_subvol(struct btree_trans *trans, u32 snapshot,
 int bch2_subvolume_get_snapshot(struct btree_trans *trans, u32 subvol,
                                u32 *snapid)
 {
-       struct bch_subvolume s;
+       struct btree_iter iter;
+       struct bkey_s_c k;
        int ret;
 
-       ret = bch2_subvolume_get_inlined(trans, subvol, true,
-                                        BTREE_ITER_CACHED|
-                                        BTREE_ITER_WITH_UPDATES,
-                                        &s);
-       if (!ret)
-               *snapid = le32_to_cpu(s.snapshot);
+       k = bch2_bkey_get_iter(trans, &iter, BTREE_ID_subvolumes, POS(0, subvol),
+                              BTREE_ITER_CACHED|
+                              BTREE_ITER_WITH_UPDATES);
+       ret = bkey_err(k) ?: k.k->type == KEY_TYPE_subvolume ? 0 : -BCH_ERR_ENOENT_subvolume;
+
+       if (likely(!ret))
+               *snapid = le32_to_cpu(bkey_s_c_to_subvolume(k).v->snapshot);
+       else if (bch2_err_matches(ret, ENOENT))
+               bch2_fs_inconsistent(trans->c, "missing subvolume %u", subvol);
+       bch2_trans_iter_exit(trans, &iter);
        return ret;
 }
 
+static int bch2_subvolume_reparent(struct btree_trans *trans,
+                                  struct btree_iter *iter,
+                                  struct bkey_s_c k,
+                                  u32 old_parent, u32 new_parent)
+{
+       struct bkey_i_subvolume *s;
+       int ret;
+
+       if (k.k->type != KEY_TYPE_subvolume)
+               return 0;
+
+       if (bkey_val_bytes(k.k) > offsetof(struct bch_subvolume, parent) &&
+           le32_to_cpu(bkey_s_c_to_subvolume(k).v->parent) != old_parent)
+               return 0;
+
+       s = bch2_bkey_make_mut_typed(trans, iter, &k, 0, subvolume);
+       ret = PTR_ERR_OR_ZERO(s);
+       if (ret)
+               return ret;
+
+       s->v.parent = cpu_to_le32(new_parent);
+       return 0;
+}
+
+/*
+ * Scan for subvolumes with parent @subvolid_to_delete, reparent:
+ */
+static int bch2_subvolumes_reparent(struct btree_trans *trans, u32 subvolid_to_delete)
+{
+       struct btree_iter iter;
+       struct bkey_s_c k;
+       struct bch_subvolume s;
+
+       return lockrestart_do(trans,
+                       bch2_subvolume_get(trans, subvolid_to_delete, true,
+                                  BTREE_ITER_CACHED, &s)) ?:
+               for_each_btree_key_commit(trans, iter,
+                               BTREE_ID_subvolumes, POS_MIN, BTREE_ITER_PREFETCH, k,
+                               NULL, NULL, BTREE_INSERT_NOFAIL,
+                       bch2_subvolume_reparent(trans, &iter, k,
+                                       subvolid_to_delete, le32_to_cpu(s.parent)));
+}
+
 /*
  * Delete subvolume, mark snapshot ID as deleted, queue up snapshot
  * deletion/cleanup:
  */
-int bch2_subvolume_delete(struct btree_trans *trans, u32 subvolid)
+static int __bch2_subvolume_delete(struct btree_trans *trans, u32 subvolid)
 {
        struct btree_iter iter;
-       struct bkey_s_c k;
        struct bkey_s_c_subvolume subvol;
        struct btree_trans_commit_hook *h;
        u32 snapid;
        int ret = 0;
 
-       bch2_trans_iter_init(trans, &iter, BTREE_ID_subvolumes,
-                            POS(0, subvolid),
-                            BTREE_ITER_CACHED|
-                            BTREE_ITER_INTENT);
-       k = bch2_btree_iter_peek_slot(&iter);
-       ret = bkey_err(k);
+       subvol = bch2_bkey_get_iter_typed(trans, &iter,
+                               BTREE_ID_subvolumes, POS(0, subvolid),
+                               BTREE_ITER_CACHED|BTREE_ITER_INTENT,
+                               subvolume);
+       ret = bkey_err(subvol);
+       bch2_fs_inconsistent_on(bch2_err_matches(ret, ENOENT), trans->c,
+                               "missing subvolume %u", subvolid);
        if (ret)
-               goto err;
-
-       if (k.k->type != KEY_TYPE_subvolume) {
-               bch2_fs_inconsistent(trans->c, "missing subvolume %u", subvolid);
-               ret = -EIO;
-               goto err;
-       }
+               return ret;
 
-       subvol = bkey_s_c_to_subvolume(k);
        snapid = le32_to_cpu(subvol.v->snapshot);
 
        ret = bch2_btree_delete_at(trans, &iter, 0);
@@ -869,7 +1570,14 @@ err:
        return ret;
 }
 
-void bch2_subvolume_wait_for_pagecache_and_delete(struct work_struct *work)
+static int bch2_subvolume_delete(struct btree_trans *trans, u32 subvolid)
+{
+       return bch2_subvolumes_reparent(trans, subvolid) ?:
+               commit_do(trans, NULL, NULL, BTREE_INSERT_NOFAIL,
+                         __bch2_subvolume_delete(trans, subvolid));
+}
+
+static void bch2_subvolume_wait_for_pagecache_and_delete(struct work_struct *work)
 {
        struct bch_fs *c = container_of(work, struct bch_fs,
                                snapshot_wait_for_pagecache_and_delete_work);
@@ -889,8 +1597,7 @@ void bch2_subvolume_wait_for_pagecache_and_delete(struct work_struct *work)
                bch2_evict_subvolume_inodes(c, &s);
 
                for (id = s.data; id < s.data + s.nr; id++) {
-                       ret = bch2_trans_do(c, NULL, NULL, BTREE_INSERT_NOFAIL,
-                                     bch2_subvolume_delete(&trans, *id));
+                       ret = bch2_trans_run(c, bch2_subvolume_delete(&trans, *id));
                        if (ret) {
                                bch_err(c, "error deleting subvolume %u: %s", *id, bch2_err_str(ret));
                                break;
@@ -908,7 +1615,7 @@ struct subvolume_unlink_hook {
        u32                             subvol;
 };
 
-int bch2_subvolume_wait_for_pagecache_and_delete_hook(struct btree_trans *trans,
+static int bch2_subvolume_wait_for_pagecache_and_delete_hook(struct btree_trans *trans,
                                                      struct btree_trans_commit_hook *_h)
 {
        struct subvolume_unlink_hook *h = container_of(_h, struct subvolume_unlink_hook, h);
@@ -926,7 +1633,7 @@ int bch2_subvolume_wait_for_pagecache_and_delete_hook(struct btree_trans *trans,
        if (!bch2_write_ref_tryget(c, BCH_WRITE_REF_snapshot_delete_pagecache))
                return -EROFS;
 
-       if (!queue_work(system_long_wq, &c->snapshot_wait_for_pagecache_and_delete_work))
+       if (!queue_work(c->write_ref_wq, &c->snapshot_wait_for_pagecache_and_delete_work))
                bch2_write_ref_put(c, BCH_WRITE_REF_snapshot_delete_pagecache);
        return 0;
 }
@@ -938,32 +1645,26 @@ int bch2_subvolume_unlink(struct btree_trans *trans, u32 subvolid)
        struct subvolume_unlink_hook *h;
        int ret = 0;
 
-       bch2_trans_iter_init(trans, &iter, BTREE_ID_subvolumes,
-                            POS(0, subvolid),
-                            BTREE_ITER_CACHED|
-                            BTREE_ITER_INTENT);
-       n = bch2_bkey_get_mut_typed(trans, &iter, subvolume);
-       ret = PTR_ERR_OR_ZERO(n);
-       if (unlikely(ret)) {
-               bch2_fs_inconsistent_on(ret == -ENOENT, trans->c, "missing subvolume %u", subvolid);
-               goto err;
-       }
-
-       SET_BCH_SUBVOLUME_UNLINKED(&n->v, true);
-
-       ret = bch2_trans_update(trans, &iter, &n->k_i, 0);
-       if (ret)
-               goto err;
-
        h = bch2_trans_kmalloc(trans, sizeof(*h));
        ret = PTR_ERR_OR_ZERO(h);
        if (ret)
-               goto err;
+               return ret;
 
        h->h.fn         = bch2_subvolume_wait_for_pagecache_and_delete_hook;
        h->subvol       = subvolid;
        bch2_trans_commit_hook(trans, &h->h);
-err:
+
+       n = bch2_bkey_get_mut_typed(trans, &iter,
+                       BTREE_ID_subvolumes, POS(0, subvolid),
+                       BTREE_ITER_CACHED, subvolume);
+       ret = PTR_ERR_OR_ZERO(n);
+       if (unlikely(ret)) {
+               bch2_fs_inconsistent_on(bch2_err_matches(ret, ENOENT), trans->c,
+                                       "missing subvolume %u", subvolid);
+               return ret;
+       }
+
+       SET_BCH_SUBVOLUME_UNLINKED(&n->v, true);
        bch2_trans_iter_exit(trans, &iter);
        return ret;
 }
@@ -978,42 +1679,28 @@ int bch2_subvolume_create(struct btree_trans *trans, u64 inode,
        struct btree_iter dst_iter, src_iter = (struct btree_iter) { NULL };
        struct bkey_i_subvolume *new_subvol = NULL;
        struct bkey_i_subvolume *src_subvol = NULL;
-       struct bkey_s_c k;
        u32 parent = 0, new_nodes[2], snapshot_subvols[2];
        int ret = 0;
 
-       for_each_btree_key(trans, dst_iter, BTREE_ID_subvolumes, SUBVOL_POS_MIN,
-                          BTREE_ITER_SLOTS|BTREE_ITER_INTENT, k, ret) {
-               if (bkey_gt(k.k->p, SUBVOL_POS_MAX))
-                       break;
-
-               /*
-                * bch2_subvolume_delete() doesn't flush the btree key cache -
-                * ideally it would but that's tricky
-                */
-               if (bkey_deleted(k.k) &&
-                   !bch2_btree_key_cache_find(c, BTREE_ID_subvolumes, dst_iter.pos))
-                       goto found_slot;
-       }
-
-       if (!ret)
+       ret = bch2_bkey_get_empty_slot(trans, &dst_iter,
+                               BTREE_ID_subvolumes, POS(0, U32_MAX));
+       if (ret == -BCH_ERR_ENOSPC_btree_slot)
                ret = -BCH_ERR_ENOSPC_subvolume_create;
-       goto err;
-found_slot:
+       if (ret)
+               return ret;
+
        snapshot_subvols[0] = dst_iter.pos.offset;
        snapshot_subvols[1] = src_subvolid;
 
        if (src_subvolid) {
                /* Creating a snapshot: */
 
-               bch2_trans_iter_init(trans, &src_iter, BTREE_ID_subvolumes,
-                                    POS(0, src_subvolid),
-                                    BTREE_ITER_CACHED|
-                                    BTREE_ITER_INTENT);
-               src_subvol = bch2_bkey_get_mut_typed(trans, &src_iter, subvolume);
+               src_subvol = bch2_bkey_get_mut_typed(trans, &src_iter,
+                               BTREE_ID_subvolumes, POS(0, src_subvolid),
+                               BTREE_ITER_CACHED, subvolume);
                ret = PTR_ERR_OR_ZERO(src_subvol);
                if (unlikely(ret)) {
-                       bch2_fs_inconsistent_on(ret == -ENOENT, trans->c,
+                       bch2_fs_inconsistent_on(bch2_err_matches(ret, ENOENT), c,
                                                "subvolume %u not found", src_subvolid);
                        goto err;
                }
@@ -1034,7 +1721,7 @@ found_slot:
                        goto err;
        }
 
-       new_subvol = bch2_bkey_alloc(trans, &dst_iter, subvolume);
+       new_subvol = bch2_bkey_alloc(trans, &dst_iter, 0, subvolume);
        ret = PTR_ERR_OR_ZERO(new_subvol);
        if (ret)
                goto err;
@@ -1042,11 +1729,12 @@ found_slot:
        new_subvol->v.flags     = 0;
        new_subvol->v.snapshot  = cpu_to_le32(new_nodes[0]);
        new_subvol->v.inode     = cpu_to_le64(inode);
+       new_subvol->v.parent    = cpu_to_le32(src_subvolid);
+       new_subvol->v.otime.lo  = cpu_to_le64(bch2_current_time(c));
+       new_subvol->v.otime.hi  = 0;
+
        SET_BCH_SUBVOLUME_RO(&new_subvol->v, ro);
        SET_BCH_SUBVOLUME_SNAP(&new_subvol->v, src_subvolid != 0);
-       ret = bch2_trans_update(trans, &dst_iter, &new_subvol->k_i, 0);
-       if (ret)
-               goto err;
 
        *new_subvolid   = new_subvol->k.p.offset;
        *new_snapshotid = new_nodes[0];