mm: PageBuddy and mapcount robustness

Change the _mapcount value indicating PageBuddy from -2 to -128 for
more robusteness against page_mapcount() undeflows.

Use reset_page_mapcount instead of __ClearPageBuddy in bad_page to
ignore the previous retval of PageBuddy().

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Reported-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 9 insertions, 2 deletions

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 679300c050f5..ff83798e1c27 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -402,16 +402,23 @@ static inline void init_page_count(struct page *page)
 /*
  * PageBuddy() indicate that the page is free and in the buddy system
  * (see mm/page_alloc.c).
+ *
+ * PAGE_BUDDY_MAPCOUNT_VALUE must be <= -2 but better not too close to
+ * -2 so that an underflow of the page_mapcount() won't be mistaken
+ * for a genuine PAGE_BUDDY_MAPCOUNT_VALUE. -128 can be created very
+ * efficiently by most CPU architectures.
  */
+#define PAGE_BUDDY_MAPCOUNT_VALUE (-128)
+
 static inline int PageBuddy(struct page *page)
 {
-        return atomic_read(&page->_mapcount) == -2;
+        return atomic_read(&page->_mapcount) == PAGE_BUDDY_MAPCOUNT_VALUE;
 }
 
 static inline void __SetPageBuddy(struct page *page)
 {
         VM_BUG_ON(atomic_read(&page->_mapcount) != -1);
-        atomic_set(&page->_mapcount, -2);
+        atomic_set(&page->_mapcount, PAGE_BUDDY_MAPCOUNT_VALUE);
 }
 
 static inline void __ClearPageBuddy(struct page *page)