bcachefs: Fix spurious "backpointer doesn't match" error in fsck

New btree nodes dont't have allocation information written until the
node has been written, which means there's a race where the backpointer
for the old node points to a node that no longer exists.

bch2_backpointer_get_node() checks for this, but fsck uses
bch2_backpointer_get_key(); this patch updates
bch2_backpointer_get_key() to call get_node() on not found to

Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>

4 files changed, 34 insertions, 16 deletions

diff --git a/fs/bcachefs/backpointers.c b/fs/bcachefs/backpointers.c
index d0b6b4f71b4c..029b1ec14283 100644
--- a/fs/bcachefs/backpointers.c
+++ b/fs/bcachefs/backpointers.c
@@ -526,9 +526,21 @@ struct bkey_s_c bch2_backpointer_get_key(struct btree_trans *trans,
 	if (extent_matches_bp(c, bp.btree_id, bp.level, k, bucket, bp))
 		return k;
 
-	backpointer_not_found(trans, bucket, bp_offset, bp, k, "extent");
-
 	bch2_trans_iter_exit(trans, iter);
+
+	if (bp.level) {
+		/*
+		 * If a backpointer for a btree node wasn't found, it may be
+		 * because it was overwritten by a new btree node that hasn't
+		 * been written out yet - backpointer_get_node() checks for
+		 * this:
+		 */
+		bch2_backpointer_get_node(trans, iter, bucket, bp_offset, bp);
+		bch2_trans_iter_exit(trans, iter);
+		return bkey_s_c_null;
+	}
+
+	backpointer_not_found(trans, bucket, bp_offset, bp, k, "extent");
 	return bkey_s_c_null;
 }
 
@@ -540,7 +552,6 @@ struct btree *bch2_backpointer_get_node(struct btree_trans *trans,
 {
 	struct bch_fs *c = trans->c;
 	struct btree *b;
-	struct bkey_s_c k;
 
 	BUG_ON(!bp.level);
 
@@ -551,22 +562,24 @@ struct btree *bch2_backpointer_get_node(struct btree_trans *trans,
 				  bp.level - 1,
 				  0);
 	b = bch2_btree_iter_peek_node(iter);
-	if (IS_ERR(b)) {
-		bch2_trans_iter_exit(trans, iter);
-		return b;
-	}
+	if (IS_ERR(b))
+		goto err;
 
 	if (extent_matches_bp(c, bp.btree_id, bp.level,
 			      bkey_i_to_s_c(&b->key),
 			      bucket, bp))
 		return b;
 
-	if (!btree_node_will_make_reachable(b))
-		backpointer_not_found(trans, bucket, bp_offset,
-				      bp, k, "btree node");
-
+	if (btree_node_will_make_reachable(b)) {
+		b = ERR_PTR(-BCH_ERR_backpointer_to_overwritten_btree_node);
+	} else {
+		backpointer_not_found(trans, bucket, bp_offset, bp,
+				      bkey_i_to_s_c(&b->key), "btree node");
+		b = NULL;
+	}
+err:
 	bch2_trans_iter_exit(trans, iter);
-	return NULL;
+	return b;
 }
 
 static int bch2_check_btree_backpointer(struct btree_trans *trans, struct btree_iter *bp_iter,
@@ -829,6 +842,8 @@ static int check_one_backpointer(struct btree_trans *trans,
 
 	k = bch2_backpointer_get_key(trans, &iter, bucket, *bp_offset, bp);
 	ret = bkey_err(k);
+	if (ret == -BCH_ERR_backpointer_to_overwritten_btree_node)
+		return 0;
 	if (ret)
 		return ret;
 
diff --git a/fs/bcachefs/btree_iter.c b/fs/bcachefs/btree_iter.c
index efc26397360b..16a7736cd6fa 100644
--- a/fs/bcachefs/btree_iter.c
+++ b/fs/bcachefs/btree_iter.c
@@ -2043,12 +2043,13 @@ inline struct bkey_s_c bch2_btree_path_peek_slot(struct btree_path *path, struct
 
 	struct bkey_s_c k;
 
+	EBUG_ON(path->uptodate != BTREE_ITER_UPTODATE);
+	EBUG_ON(!btree_node_locked(path, path->level));
+
 	if (!path->cached) {
 		struct btree_path_level *l = path_l(path);
 		struct bkey_packed *_k;
 
-		EBUG_ON(path->uptodate != BTREE_ITER_UPTODATE);
-
 		_k = bch2_btree_node_iter_peek_all(&l->iter, l->b);
 		k = _k ? bkey_disassemble(l->b, _k, u) : bkey_s_c_null;
 
@@ -2063,7 +2064,6 @@ inline struct bkey_s_c bch2_btree_path_peek_slot(struct btree_path *path, struct
 			(path->btree_id != ck->key.btree_id ||
 			 bkey_cmp(path->pos, ck->key.pos)));
 		EBUG_ON(!ck || !ck->valid);
-		EBUG_ON(path->uptodate != BTREE_ITER_UPTODATE);
 
 		*u = ck->k->k;
 		k = bkey_i_to_s_c(ck->k);
@@ -2896,7 +2896,7 @@ struct bkey_s_c bch2_btree_iter_peek_slot(struct btree_iter *iter)
 		}
 
 		if (unlikely(bkey_err(k)))
-			return k;
+			goto out_no_locked;
 
 		next = k.k ? bkey_start_pos(k.k) : POS_MAX;
 
diff --git a/fs/bcachefs/errcode.h b/fs/bcachefs/errcode.h
index 15a1be2fcc84..232f7c7999f6 100644
--- a/fs/bcachefs/errcode.h
+++ b/fs/bcachefs/errcode.h
@@ -37,6 +37,7 @@
 	x(no_btree_node,	no_btree_node_down)			\
 	x(no_btree_node,	no_btree_node_init)			\
 	x(no_btree_node,	no_btree_node_cached)			\
+	x(0,			backpointer_to_overwritten_btree_node)	\
 	x(0,			lock_fail_node_reused)			\
 	x(0,			lock_fail_root_changed)			\
 	x(0,			journal_reclaim_would_deadlock)		\
diff --git a/fs/bcachefs/move.c b/fs/bcachefs/move.c
index 2fc247451390..224700675604 100644
--- a/fs/bcachefs/move.c
+++ b/fs/bcachefs/move.c
@@ -636,6 +636,8 @@ int __bch2_evacuate_bucket(struct moving_context *ctxt,
 			b = bch2_backpointer_get_node(&trans, &iter,
 						bucket, bp_offset, bp);
 			ret = PTR_ERR_OR_ZERO(b);
+			if (ret == -BCH_ERR_backpointer_to_overwritten_btree_node)
+				continue;
 			if (bch2_err_matches(ret, BCH_ERR_transaction_restart))
 				continue;
 			if (ret)