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authorAndrea Arcangeli <aarcange@redhat.com>2011-03-18 00:16:35 +0100
committerLinus Torvalds <torvalds@linux-foundation.org>2011-03-17 16:31:13 -0700
commitef2b4b95a63a1d23958dcb99eb2c6898eddc87d0 (patch)
tree0a22745dba03926768552aa3006b166995e5342d /include
parent7b7adc4a016a1decb806eb71ecab98721fa7f146 (diff)
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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>
Diffstat (limited to 'include')
-rw-r--r--include/linux/mm.h11
1 files changed, 9 insertions, 2 deletions
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 679300c050f..ff83798e1c2 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)