bcachefs: Convert snapshot table to RCU array

This switches the generic radix tree for the in-memory table of snapshot
nodes to a simple rcu array. This means we have to add new locking to
deal with reallocations, but is faster than traversing the radix tree.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/bcachefs.h

@@ -774,9 +774,10 @@ struct bch_fs {
 	struct mutex		sb_lock;
 
 	/* snapshot.c: */
-	GENRADIX(struct snapshot_t) snapshots;
-	struct bch_snapshot_table __rcu *snapshot_table;
+	struct snapshot_table __rcu *snapshots;
+	size_t			snapshot_table_size;
 	struct mutex		snapshot_table_lock;
+
 	struct work_struct	snapshot_delete_work;
 	struct work_struct	snapshot_wait_for_pagecache_and_delete_work;
 	snapshot_id_list	snapshots_unlinked;

fs/bcachefs/btree_update_leaf.c

@@ -311,7 +311,7 @@ static inline void btree_insert_entry_checks(struct btree_trans *trans,
 		!(i->flags & BTREE_UPDATE_INTERNAL_SNAPSHOT_NODE) &&
 		test_bit(JOURNAL_REPLAY_DONE, &trans->c->journal.flags) &&
 		i->k->k.p.snapshot &&
-		bch2_snapshot_internal_node(trans->c, i->k->k.p.snapshot));
+		bch2_snapshot_is_internal_node(trans->c, i->k->k.p.snapshot));
 }
 
 static noinline int
@@ -1229,7 +1229,7 @@ static inline int check_pos_snapshot_overwritten(struct btree_trans *trans,
 					  struct bpos pos)
 {
 	if (!btree_type_has_snapshots(id) ||
-	    !snapshot_t(trans->c, pos.snapshot)->children[0])
+	    bch2_snapshot_is_leaf(trans->c, pos.snapshot))
 		return 0;
 
 	return __check_pos_snapshot_overwritten(trans, id, pos);

fs/bcachefs/fsck.c

@@ -894,7 +894,7 @@ static int check_inode(struct btree_trans *trans,
 	 * particular is not atomic, so on the internal snapshot nodes
 	 * we can see inodes marked for deletion after a clean shutdown
 	 */
-	if (bch2_snapshot_internal_node(c, k.k->p.snapshot))
+	if (bch2_snapshot_is_internal_node(c, k.k->p.snapshot))
 		return 0;
 
 	if (!bkey_is_inode(k.k))

fs/bcachefs/quota.c

@@ -562,7 +562,7 @@ static int bch2_fs_quota_read_inode(struct btree_trans *trans,
 	int ret;
 
 	ret = bch2_snapshot_tree_lookup(trans,
-			snapshot_t(c, k.k->p.snapshot)->tree, &s_t);
+			bch2_snapshot_tree(c, k.k->p.snapshot), &s_t);
 	bch2_fs_inconsistent_on(bch2_err_matches(ret, ENOENT), c,
 			"%s: snapshot tree %u not found", __func__,
 			snapshot_t(c, k.k->p.snapshot)->tree);

fs/bcachefs/subvolume.c

@@ -12,9 +12,9 @@
 
 static int bch2_subvolume_delete(struct btree_trans *, u32);
 
-static inline u32 get_ancestor_below(struct bch_fs *c, u32 id, u32 ancestor)
+static inline u32 get_ancestor_below(struct snapshot_table *t, u32 id, u32 ancestor)
 {
-	struct snapshot_t *s = snapshot_t(c, id);
+	const struct snapshot_t *s = __snapshot_t(t, id);
 
 	if (s->skip[2] <= ancestor)
 		return s->skip[2];
@@ -27,22 +27,102 @@ static inline u32 get_ancestor_below(struct bch_fs *c, u32 id, u32 ancestor)
 
 bool bch2_snapshot_is_ancestor(struct bch_fs *c, u32 id, u32 ancestor)
 {
+	struct snapshot_table *t;
+
 	EBUG_ON(c->curr_recovery_pass <= BCH_RECOVERY_PASS_check_snapshots);
 
+	rcu_read_lock();
+	t = rcu_dereference(c->snapshots);
+
 	while (id && id < ancestor)
-		id = get_ancestor_below(c, id, ancestor);
+		id = get_ancestor_below(t, id, ancestor);
+	rcu_read_unlock();
 
 	return id == ancestor;
 }
 
 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(c, id)->parent;
+		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;
+}
+
+struct snapshot_t_free_rcu {
+	struct rcu_head		rcu;
+	struct snapshot_table	*t;
+};
+
+static void snapshot_t_free_rcu(struct rcu_head *rcu)
+{
+	struct snapshot_t_free_rcu *free_rcu =
+		container_of(rcu, struct snapshot_t_free_rcu, rcu);
+
+	kvfree(free_rcu->t);
+	kfree(free_rcu);
+}
+
+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) {
+		struct snapshot_t_free_rcu *rcu =
+			kmalloc(sizeof(*rcu), GFP_KERNEL|__GFP_NOFAIL);
+
+		rcu->t = old;
+		call_rcu(&rcu->rcu, snapshot_t_free_rcu);
+	}
+
+	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,
@@ -209,12 +289,15 @@ int bch2_mark_snapshot(struct btree_trans *trans,
 {
 	struct bch_fs *c = trans->c;
 	struct snapshot_t *t;
+	int ret = 0;
 
-	t = genradix_ptr_alloc(&c->snapshots,
-			       U32_MAX - new.k->p.offset,
-			       GFP_KERNEL);
-	if (!t)
-		return -BCH_ERR_ENOMEM_mark_snapshot;
+	mutex_lock(&c->snapshot_table_lock);
+
+	t = snapshot_t_mut(c, new.k->p.offset);
+	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);
@@ -246,8 +329,9 @@ int bch2_mark_snapshot(struct btree_trans *trans,
 		t->subvol	= 0;
 		t->tree		= 0;
 	}
-
-	return 0;
+err:
+	mutex_unlock(&c->snapshot_table_lock);
+	return ret;
 }
 
 static int snapshot_lookup(struct btree_trans *trans, u32 id,
@@ -300,9 +384,14 @@ 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;
 }
 
@@ -520,16 +609,18 @@ static int snapshot_tree_ptr_good(struct btree_trans *trans,
 
 static u32 snapshot_skiplist_get(struct bch_fs *c, u32 id)
 {
-	struct snapshot_t *s;
+	const struct snapshot_t *s;
 
 	if (!id)
 		return 0;
 
+	rcu_read_lock();
 	s = snapshot_t(c, id);
-	if (!s->parent)
-		return id;
+	if (s->parent)
+		id = bch2_snapshot_nth_parent(c, id, get_random_u32_below(s->depth));
+	rcu_read_unlock();
 
-	return bch2_snapshot_nth_parent(c, id, get_random_u32_below(s->depth));
+	return id;
 }
 
 static int snapshot_skiplist_good(struct btree_trans *trans, struct bch_snapshot s)
@@ -633,9 +724,6 @@ static int check_snapshot(struct btree_trans *trans,
 	struct bkey_i_snapshot *u;
 	u32 parent_id = bch2_snapshot_parent_early(c, k.k->p.offset);
 	u32 real_depth;
-	struct snapshot_t *parent = parent_id
-		? snapshot_t(c, parent_id)
-		: NULL;
 	struct printbuf buf = PRINTBUF;
 	bool should_have_subvol;
 	u32 i, id;
@@ -726,7 +814,7 @@ static int check_snapshot(struct btree_trans *trans,
 	}
 	ret = 0;
 
-	real_depth = parent ? parent->depth + 1 : 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 ||
@@ -823,9 +911,13 @@ static int check_subvol(struct btree_trans *trans,
 
 	if (!BCH_SUBVOLUME_SNAP(subvol.v)) {
 		u32 snapshot_root = bch2_snapshot_root(c, le32_to_cpu(subvol.v->snapshot));
-		u32 snapshot_tree = snapshot_t(c, snapshot_root)->tree;
+		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,
@@ -869,7 +961,7 @@ int bch2_check_subvols(struct bch_fs *c)
 
 void bch2_fs_snapshots_exit(struct bch_fs *c)
 {
-	genradix_free(&c->snapshots);
+	kfree(rcu_dereference_protected(c->snapshots, true));
 }
 
 int bch2_snapshots_read(struct bch_fs *c)
@@ -1011,7 +1103,7 @@ static int create_snapids(struct btree_trans *trans, u32 parent, u32 tree,
 	struct bkey_i_snapshot *n;
 	struct bkey_s_c k;
 	unsigned i, j;
-	u32 depth = parent ? snapshot_t(c, parent)->depth + 1 : 0;
+	u32 depth = bch2_snapshot_depth(c, parent);
 	int ret;
 
 	bch2_trans_iter_init(trans, &iter, BTREE_ID_snapshots,
@@ -1150,7 +1242,7 @@ 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;

fs/bcachefs/subvolume.h

@@ -32,17 +32,40 @@ int bch2_mark_snapshot(struct btree_trans *, enum btree_id, unsigned,
 	.min_val_size	= 24,					\
 })
 
-static inline struct snapshot_t *snapshot_t(struct bch_fs *c, u32 id)
+static inline struct snapshot_t *__snapshot_t(struct snapshot_table *t, u32 id)
 {
-	return genradix_ptr(&c->snapshots, U32_MAX - id);
+	return &t->s[U32_MAX - id];
 }
 
-static inline u32 bch2_snapshot_parent_early(struct bch_fs *c, u32 id)
+static inline const struct snapshot_t *snapshot_t(struct bch_fs *c, u32 id)
+{
+	return __snapshot_t(rcu_dereference(c->snapshots), id);
+}
+
+static inline u32 bch2_snapshot_tree(struct bch_fs *c, u32 id)
+{
+	rcu_read_lock();
+	id = snapshot_t(c, id)->tree;
+	rcu_read_unlock();
+
+	return id;
+}
+
+static inline u32 __bch2_snapshot_parent_early(struct bch_fs *c, u32 id)
 {
 	return snapshot_t(c, id)->parent;
 }
 
-static inline u32 bch2_snapshot_parent(struct bch_fs *c, u32 id)
+static inline u32 bch2_snapshot_parent_early(struct bch_fs *c, u32 id)
+{
+	rcu_read_lock();
+	id = __bch2_snapshot_parent_early(c, id);
+	rcu_read_unlock();
+
+	return id;
+}
+
+static inline u32 __bch2_snapshot_parent(struct bch_fs *c, u32 id)
 {
 #ifdef CONFIG_BCACHEFS_DEBUG
 	u32 parent = snapshot_t(c, id)->parent;
@@ -59,10 +82,21 @@ static inline u32 bch2_snapshot_parent(struct bch_fs *c, u32 id)
 #endif
 }
 
+static inline u32 bch2_snapshot_parent(struct bch_fs *c, u32 id)
+{
+	rcu_read_lock();
+	id = __bch2_snapshot_parent(c, id);
+	rcu_read_unlock();
+
+	return id;
+}
+
 static inline u32 bch2_snapshot_nth_parent(struct bch_fs *c, u32 id, u32 n)
 {
+	rcu_read_lock();
 	while (n--)
-		id = bch2_snapshot_parent(c, id);
+		id = __bch2_snapshot_parent(c, id);
+	rcu_read_unlock();
 
 	return id;
 }
@@ -71,37 +105,60 @@ static inline u32 bch2_snapshot_root(struct bch_fs *c, u32 id)
 {
 	u32 parent;
 
-	while ((parent = bch2_snapshot_parent(c, id)))
+	rcu_read_lock();
+	while ((parent = __bch2_snapshot_parent(c, id)))
 		id = parent;
+	rcu_read_unlock();
+
 	return id;
 }
 
-static inline u32 bch2_snapshot_equiv(struct bch_fs *c, u32 id)
+static inline u32 __bch2_snapshot_equiv(struct bch_fs *c, u32 id)
 {
 	return snapshot_t(c, id)->equiv;
 }
 
+static inline u32 bch2_snapshot_equiv(struct bch_fs *c, u32 id)
+{
+	rcu_read_lock();
+	id = __bch2_snapshot_equiv(c, id);
+	rcu_read_unlock();
+
+	return id;
+}
+
 static inline bool bch2_snapshot_is_equiv(struct bch_fs *c, u32 id)
 {
-	return id == snapshot_t(c, id)->equiv;
+	return id == bch2_snapshot_equiv(c, id);
 }
 
-static inline u32 bch2_snapshot_internal_node(struct bch_fs *c, u32 id)
+static inline bool bch2_snapshot_is_internal_node(struct bch_fs *c, u32 id)
 {
-	struct snapshot_t *s = snapshot_t(c, id);
+	const struct snapshot_t *s;
+	bool ret;
+
+	rcu_read_lock();
+	s = snapshot_t(c, id);
+	ret = s->children[0];
+	rcu_read_unlock();
 
-	return s->children[0] || s->children[1];
+	return ret;
+}
+
+static inline u32 bch2_snapshot_is_leaf(struct bch_fs *c, u32 id)
+{
+	return !bch2_snapshot_is_internal_node(c, id);
 }
 
 static inline u32 bch2_snapshot_sibling(struct bch_fs *c, u32 id)
 {
-	struct snapshot_t *s;
-	u32 parent = bch2_snapshot_parent(c, id);
+	const struct snapshot_t *s;
+	u32 parent = __bch2_snapshot_parent(c, id);
 
 	if (!parent)
 		return 0;
 
-	s = snapshot_t(c, bch2_snapshot_parent(c, id));
+	s = snapshot_t(c, __bch2_snapshot_parent(c, id));
 	if (id == s->children[0])
 		return s->children[1];
 	if (id == s->children[1])
@@ -113,9 +170,15 @@ bool bch2_snapshot_is_ancestor(struct bch_fs *, u32, u32);
 
 static inline bool bch2_snapshot_has_children(struct bch_fs *c, u32 id)
 {
-	struct snapshot_t *t = snapshot_t(c, id);
+	const struct snapshot_t *t;
+	bool ret;
 
-	return (t->children[0]|t->children[1]) != 0;
+	rcu_read_lock();
+	t = snapshot_t(c, id);
+	ret = (t->children[0]|t->children[1]) != 0;
+	rcu_read_unlock();
+
+	return ret;
 }
 
 static inline bool snapshot_list_has_id(snapshot_id_list *s, u32 id)

fs/bcachefs/subvolume_types.h

@@ -16,6 +16,10 @@ struct snapshot_t {
 	u32			equiv;
 };
 
+struct snapshot_table {
+	struct snapshot_t	s[0];
+};
+
 typedef struct {
 	u32		subvol;
 	u64		inum;