[PATCH v2 1/4] mm: correct mask size for slub page->objects

From: Yu Zhao <yuzhao@google.com>
To: Christoph Lameter <cl@linux.com>,
	Pekka Enberg <penberg@kernel.org>,
	David Rientjes <rientjes@google.com>,
	Joonsoo Kim <iamjoonsoo.kim@lge.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	"Kirill A . Shutemov" <kirill@shutemov.name>,
	Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	Yu Zhao <yuzhao@google.com>
Subject: [PATCH v2 1/4] mm: correct mask size for slub page->objects
Date: Wed, 11 Sep 2019 20:31:08 -0600	[thread overview]
Message-ID: <20190912023111.219636-1-yuzhao@google.com> (raw)
In-Reply-To: <20190912004401.jdemtajrspetk3fh@box>

Mask of slub objects per page shouldn't be larger than what
page->objects can hold.

It requires more than 2^15 objects to hit the problem, and I don't
think anybody would. It'd be nice to have the mask fixed, but not
really worth cc'ing the stable.

Fixes: 50d5c41cd151 ("slub: Do not use frozen page flag but a bit in the page counters")
Signed-off-by: Yu Zhao <yuzhao@google.com>
---
 mm/slub.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/mm/slub.c b/mm/slub.c
index 8834563cdb4b..62053ceb4464 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -187,7 +187,7 @@ static inline bool kmem_cache_has_cpu_partial(struct kmem_cache *s)
  */
 #define DEBUG_METADATA_FLAGS (SLAB_RED_ZONE | SLAB_POISON | SLAB_STORE_USER)
 
-#define OO_SHIFT	16
+#define OO_SHIFT	15
 #define OO_MASK		((1 << OO_SHIFT) - 1)
 #define MAX_OBJS_PER_PAGE	32767 /* since page.objects is u15 */
 
@@ -343,6 +343,8 @@ static inline unsigned int oo_order(struct kmem_cache_order_objects x)
 
 static inline unsigned int oo_objects(struct kmem_cache_order_objects x)
 {
+	BUILD_BUG_ON(OO_MASK > MAX_OBJS_PER_PAGE);
+
 	return x.x & OO_MASK;
 }
 
-- 
2.23.0.162.g0b9fbb3734-goog

