diff --git a/lib/radix-tree.c b/lib/radix-tree.c
index d7c878c..e796429 100644
--- a/lib/radix-tree.c
+++ b/lib/radix-tree.c
@@ -686,6 +686,9 @@
 	 * during iterating; it can be zero only at the beginning.
 	 * And we cannot overflow iter->next_index in a single step,
 	 * because RADIX_TREE_MAP_SHIFT < BITS_PER_LONG.
+	 *
+	 * This condition also used by radix_tree_next_slot() to stop
+	 * contiguous iterating, and forbid swithing to the next chunk.
 	 */
 	index = iter->next_index;
 	if (!index && iter->index)