[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>

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 85854b8..75e9f07 100644 (file)
--- 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 bc2442a..7624742 100644 (file)
--- 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;