diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 0e309cd1b5b9..793e9874de51 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -305,7 +305,9 @@ static unsigned long task_min_dirty_limit(unsigned long bdi_dirty)
 }
 
 /*
- *
+ * bdi_min_ratio keeps the sum of the minimum dirty shares of all
+ * registered backing devices, which, for obvious reasons, can not
+ * exceed 100%.
  */
 static unsigned int bdi_min_ratio;