diff --git a/kmod/src/btree.c b/kmod/src/btree.c
index 1b83c498..603f4110 100644
--- a/kmod/src/btree.c
+++ b/kmod/src/btree.c
@@ -86,6 +86,7 @@ enum btree_walk_flags {
 	 BTW_PAR_RNG	= (1 <<  6), /* return range through final parent */
 	 BTW_GET_PAR	= (1 <<  7), /* get reference to final parent */
 	 BTW_SET_PAR	= (1 <<  8), /* override reference to final parent */
+	 BTW_SUBTREE	= (1 <<  9), /* root is parent subtree, return -ERANGE if split/join */
 };
 
 /* total length of the value payload */
@@ -1209,6 +1210,17 @@ restart:
 			break;
 		}
 
+		/*
+		 * join/split won't check subtree parent root, let
+		 * caller know when it needs to be split/join.
+		 */
+		if ((flags & BTW_SUBTREE) && level == 1 &&
+		    (!total_above_join_low_water(bt) ||
+		     !mid_free_item_room(bt, sizeof(struct scoutfs_block_ref)))) {
+			ret = -ERANGE;
+			break;
+		}
+
 		/*
 		 * Splitting and joining can add or remove parents or
 		 * change the parent item we use to reach the child