Merge remote-tracking branch 'linux-stable/linux-4.15.y' into bcachefs-buffered-io-2

1 files changed, 11 insertions, 1 deletions

diff --git a/fs/btrfs/backref.c b/fs/btrfs/backref.c
index 7d0dc100a09a..8a9df8003345 100644
--- a/fs/btrfs/backref.c
+++ b/fs/btrfs/backref.c
@@ -1263,7 +1263,16 @@ again:
 	while (node) {
 		ref = rb_entry(node, struct prelim_ref, rbnode);
 		node = rb_next(&ref->rbnode);
-		WARN_ON(ref->count < 0);
+		/*
+		 * ref->count < 0 can happen here if there are delayed
+		 * refs with a node->action of BTRFS_DROP_DELAYED_REF.
+		 * prelim_ref_insert() relies on this when merging
+		 * identical refs to keep the overall count correct.
+		 * prelim_ref_insert() will merge only those refs
+		 * which compare identically.  Any refs having
+		 * e.g. different offsets would not be merged,
+		 * and would retain their original ref->count < 0.
+		 */
 		if (roots && ref->count && ref->root_id && ref->parent == 0) {
 			if (sc && sc->root_objectid &&
 			    ref->root_id != sc->root_objectid) {
@@ -1509,6 +1518,7 @@ int btrfs_check_shared(struct btrfs_root *root, u64 inum, u64 bytenr)
 		if (!node)
 			break;
 		bytenr = node->val;
+		shared.share_count = 0;
 		cond_resched();
 	}