diff --git a/fs/btrfs/backref.c b/fs/btrfs/backref.c
index e4054e533f6d..f94b2d8c744a 100644
--- a/fs/btrfs/backref.c
+++ b/fs/btrfs/backref.c
@@ -1264,7 +1264,16 @@ 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) {