[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>

2 files changed, 23 insertions, 11 deletions

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 85854b867463..75e9f0724997 100644
--- a/include/linux/mm.h
+++ b/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);  \
         })
 
diff --git a/mm/swap.c b/mm/swap.c
index bc2442a7b0ee..76247424dea1 100644
--- a/mm/swap.c
+++ b/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;