diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 955fe35d01e05da983fff97a0cc0902d3425774b..b8be62381396a296ba6669c7510e85bbf542483d 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -437,10 +437,17 @@ void global_dirty_limits(unsigned long *pbackground, unsigned long *pdirty)
 	*pdirty = dirty;
 }
 
-/*
+/**
  * bdi_dirty_limit - @bdi's share of dirty throttling threshold
+ * @bdi: the backing_dev_info to query
+ * @dirty: global dirty limit in pages
+ *
+ * Returns @bdi's dirty limit in pages. The term "dirty" in the context of
+ * dirty balancing includes all PG_dirty, PG_writeback and NFS unstable pages.
+ * And the "limit" in the name is not seriously taken as hard limit in
+ * balance_dirty_pages().
  *
- * Allocate high/low dirty limits to fast/slow devices, in order to prevent
+ * It allocates high/low dirty limits to fast/slow devices, in order to prevent
  * - starving fast devices
  * - piling up dirty pages (that will take long time to sync) on slow devices
  *