diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 28763b8bdbddd9caba1478cc9e8aa205a02cf397..2cb01f6ec5d019e7e431d9f18cdba6f3bde573e4 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -410,9 +410,12 @@ void global_dirty_limits(unsigned long *pbackground, unsigned long *pdirty)
 {
 	unsigned long background;
 	unsigned long dirty;
-	unsigned long available_memory = determine_dirtyable_memory();
+	unsigned long uninitialized_var(available_memory);
 	struct task_struct *tsk;
 
+	if (!vm_dirty_bytes || !dirty_background_bytes)
+		available_memory = determine_dirtyable_memory();
+
 	if (vm_dirty_bytes)
 		dirty = DIV_ROUND_UP(vm_dirty_bytes, PAGE_SIZE);
 	else