[PATCH 2/2] mm: Fix slab->page _count corruption.

[PATCH 2/2] 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@nicira.com>
Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
---
 include/linux/mm_types.h |   25 ++++++++++++++++++++++++-
 1 file changed, 24 insertions(+), 1 deletion(-)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index dad95bd..7f0032f 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -55,7 +55,8 @@ struct page {
 			pgoff_t index;		/* Our offset within mapping. */
 			void *freelist;		/* slub first free object */
 		};
-
+#if defined(CONFIG_HAVE_CMPXCHG_DOUBLE) && \
+    defined(CONFIG_HAVE_ALIGNED_STRUCT_PAGE)
 		union {
 			/* Used for cmpxchg_double in slub */
 			unsigned long counters;
@@ -90,6 +91,28 @@ struct page {
 				atomic_t _count;		/* Usage count, see below. */
 			};
 		};
+#else
+		/* Keep _count separate from slub cmpxchg_double data, As rest
+		 * of double word is protected by slab_lock but _count is not */
+		union {
+			/* Used for cmpxchg_double in slub */
+			unsigned int counters;
+
+			struct {
+
+				union {
+					atomic_t _mapcount;
+
+					struct {
+						unsigned inuse:16;
+						unsigned objects:15;
+						unsigned frozen:1;
+					};
+				};
+			};
+		};
+		atomic_t _count;
+#endif
 	};
 
 	/* Third double word block */
-- 
1.7.10

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Re: [PATCH 2/2] mm: Fix slab->page _count corruption.

[Christoph Lameter]

On Mon, 14 May 2012, Pravin B Shelar wrote:

> 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.

Ugly. Maybe its best to not touch the count in the page lock case in slub?

You could accomplish that by changing the definition of counters in
mm_types.h. Make it unsigned instead of unsigned long so that it only
covers the first part of the struct (which excludes the refcounter)


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