--- ./mm/rmap.c.orig	2008-02-01 15:36:50.000000000 +0900
+++ ./mm/rmap.c	2008-02-01 15:42:43.000000000 +0900
@@ -276,6 +276,8 @@ static int page_referenced_one(struct pa
 	spinlock_t *ptl;
 	int referenced = 0;
 
+	BUG_ON(( mm == NULL ));
+
 	address = vma_address(page, vma);
 	if (address == -EFAULT)
 		goto out;