[PATCH] mm: compound release fix

Compound pages on SMP systems can now often be freed from pagetables via
the release_pages path.  This uses put_page_testzero which does not handle
compound pages at all.  Releasing constituent pages from process mappings
decrements their count to a large negative number and leaks the reference
at the head page - net result is a memory leak.

The problem was hidden because the debug check in put_page_testzero itself
actually did take compound pages into consideration.

Fix the bug and the debug check.

Signed-off-by: Nick Piggin <npiggin@suse.de>
Acked-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

include/linux/mm.h

@@ -303,7 +303,7 @@ struct page {
  */
 #define put_page_testzero(p)				\
 	({						\
-		BUG_ON(page_count(p) == 0);		\
+		BUG_ON(atomic_read(&(p)->_count) == -1);\
 		atomic_add_negative(-1, &(p)->_count);	\
 	})
 

mm/swap.c

@@ -34,19 +34,22 @@
 /* How many pages do we try to swap or page in/out together? */
 int page_cluster;
 
-void put_page(struct page *page)
+static void put_compound_page(struct page *page)
 {
-	if (unlikely(PageCompound(page))) {
-		page = (struct page *)page_private(page);
-		if (put_page_testzero(page)) {
-			void (*dtor)(struct page *page);
+	page = (struct page *)page_private(page);
+	if (put_page_testzero(page)) {
+		void (*dtor)(struct page *page);
 
-			dtor = (void (*)(struct page *))page[1].mapping;
-			(*dtor)(page);
-		}
-		return;
+		dtor = (void (*)(struct page *))page[1].mapping;
+		(*dtor)(page);
 	}
-	if (put_page_testzero(page))
+}
+
+void put_page(struct page *page)
+{
+	if (unlikely(PageCompound(page)))
+		put_compound_page(page);
+	else if (put_page_testzero(page))
 		__page_cache_release(page);
 }
 EXPORT_SYMBOL(put_page);
@@ -244,6 +247,15 @@ void release_pages(struct page **pages, int nr, int cold)
 		struct page *page = pages[i];
 		struct zone *pagezone;
 
+		if (unlikely(PageCompound(page))) {
+			if (zone) {
+				spin_unlock_irq(&zone->lru_lock);
+				zone = NULL;
+			}
+			put_compound_page(page);
+			continue;
+		}
+
 		if (!put_page_testzero(page))
 			continue;