[PATCH] fix missing unlock_page in mm/rmap.c

A required unlock_page will be missed in a very rare (but possible) race
condition. Acked by Hugh, who says:

	It'll be hard to hit because of the additional page_mapped test above,
	with truncate unmapping ptes from mms before it advances to removing
	pages from cache; but nothing to prevent it happening.
Signed-off-by: Nick Piggin <nickpiggin@yahoo.com.au>
Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

mm/rmap.c

@@ -405,8 +405,9 @@ int page_referenced(struct page *page, int is_locked)
 			referenced += page_referenced_file(page);
 		else if (TestSetPageLocked(page))
 			referenced++;
-		else if (page->mapping) {
-			referenced += page_referenced_file(page);
+		else {
+			if (page->mapping)
+				referenced += page_referenced_file(page);
 			unlock_page(page);
 		}
 	}