[PATCH] slab : allow SLAB_RED_ZONE and SLAB_STORE_USER to work on arm

The current slab code only allow to put redzone( and user store) info if "buffer
alignment(ralign) <= __alignof__(unsigned long long)". This was done because we
want to keep the buffer aligned for user even after adding redzone@the
beginning of the buffer (the user store is stored after user buffer) [1]

But instead of disabling this feature when "ralign > __alignof__(unsigned long
long)", we can force the alignment by allocating ralign before user buffer.

This is done by setting ralign in obj_offset when "ralign > __alignof__(unsigned
long long)" and keeping the old behavior when  "ralign <= __alignof__(unsigned
long long)" (we set sizeof(unsigned long long)).

The 5c5e3b33b7cb959a401f823707bee006caadd76e commit wasn't handling "ralign <=
__alignof__(unsigned long long)", that's why it broked some configuration.

This was tested on omap3 (ralign is 64 and __alignof__(unsigned long long) is 8)

[1]
/*
 * memory layout of objects:
 * 0        : objp
 * 0 .. cachep->obj_offset - BYTES_PER_WORD - 1: padding. This ensures that
 *      the end of an object is aligned with the end of the real
 *      allocation. Catches writes behind the end of the allocation.
 * cachep->obj_offset - BYTES_PER_WORD .. cachep->obj_offset - 1:
 *      redzone word.
 * cachep->obj_offset: The real object.
 * cachep->buffer_size - 2* BYTES_PER_WORD: redzone word [BYTES_PER_WORD long]
 * cachep->buffer_size - 1* BYTES_PER_WORD: last caller address
 *                  [BYTES_PER_WORD long]
 */

Signed-off-by: Matthieu Castet <redacted>
CC: Russell King <redacted>
CC: Pekka Enberg <redacted>
---
 mm/slab.c |    8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

diff --git a/mm/slab.c b/mm/slab.c
index e7667a3..d2380d9 100644
--- a/mm/slab.c
+++ b/mm/slab.c

@@ -2392,9 +2392,6 @@ __kmem_cache_create (struct kmem_cache *cachep, unsigned long flags)
 	if (ralign < cachep->align) {
 		ralign = cachep->align;
 	}
-	/* disable debug if necessary */
-	if (ralign > __alignof__(unsigned long long))
-		flags &= ~(SLAB_RED_ZONE | SLAB_STORE_USER);
 	/*
 	 * 4) Store it.
 	 */

@@ -2414,8 +2411,9 @@ __kmem_cache_create (struct kmem_cache *cachep, unsigned long flags)
 	 */
 	if (flags & SLAB_RED_ZONE) {
 		/* add space for red zone words */
-		cachep->obj_offset += sizeof(unsigned long long);
-		size += 2 * sizeof(unsigned long long);
+		int offset = max(ralign, sizeof(unsigned long long));
+		cachep->obj_offset += offset;
+		size += offset + sizeof(unsigned long long);
 	}
 	if (flags & SLAB_STORE_USER) {
 		/* user store requires one word storage behind the end of

-- 
1.7.10.4