[PATCH v2] mm: Fix slab->page _count corruption.

[Pravin B Shelar]

On arches that do not support this_cpu_cmpxchg_double slab_lock is used
to do atomic cmpxchg() on double word which contains page->_count.
page count can be changed from get_page() or put_page() without taking
slab_lock. That corrupts page counter.

Following patch fixes it by moving page->_count out of cmpxchg_double
data. So that slub does no change it while updating slub meta-data in
struct page.

Reported-by: Amey Bhide <abhide@xxxxxxxxxx>
Signed-off-by: Pravin B Shelar <pshelar@xxxxxxxxxx>
---
 include/linux/mm_types.h |    8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index dad95bd..5f558dc 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -57,8 +57,16 @@ struct page {
 		};
 
 		union {
+#if defined(CONFIG_HAVE_CMPXCHG_DOUBLE) && \
+    defined(CONFIG_HAVE_ALIGNED_STRUCT_PAGE)
 			/* Used for cmpxchg_double in slub */
 			unsigned long counters;
+#else
+			/* Keep _count separate from slub cmpxchg_double data,
+			 * As rest of double word is protected by slab_lock
+			 * but _count is not. */
+			unsigned counters;
+#endif
 
 			struct {
 
-- 
1.7.10

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