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SubjectRe: [patch 2/2] slab: always consider arch mandated alignment
On Wed, 26 Jul 2006, Heiko Carstens wrote:
> We only specify ARCH_KMALLOC_MINALIGN, since that aligns only the kmalloc
> caches, but it doesn't disable debugging on other caches that are created
> via kmem_cache_create() where an alignment of e.g. 0 is specified.
>
> The point of the first patch is: why should the slab cache be allowed to chose
> an aligment that is less than what the caller specified? This does very likely
> break things.

Ah, yes, you are absolutely right. We need to respect caller mandated
alignment too. How about this?

Pekka

[PATCH] slab: respect architecture and caller mandated alignment

Ensure cache alignment is always at minimum what the architecture or
caller mandates even if slab debugging is enabled.

Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>
---
diff --git a/mm/slab.c b/mm/slab.c
index 0f20843..3767460 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -2097,6 +2097,15 @@ #endif
} else {
ralign = BYTES_PER_WORD;
}
+
+ /*
+ * Redzoning and user store require word alignment. Note this will be
+ * overridden by architecture or caller mandated alignment if either
+ * is greater than BYTES_PER_WORD.
+ */
+ if (flags & SLAB_RED_ZONE || flags & SLAB_STORE_USER)
+ ralign = BYTES_PER_WORD;
+
/* 2) arch mandated alignment: disables debug if necessary */
if (ralign < ARCH_SLAB_MINALIGN) {
ralign = ARCH_SLAB_MINALIGN;
@@ -2123,20 +2132,19 @@ #endif
#if DEBUG
cachep->obj_size = size;

+ /*
+ * Both debugging options require word-alignment which is calculated
+ * into align above.
+ */
if (flags & SLAB_RED_ZONE) {
- /* redzoning only works with word aligned caches */
- align = BYTES_PER_WORD;
-
/* add space for red zone words */
cachep->obj_offset += BYTES_PER_WORD;
size += 2 * BYTES_PER_WORD;
}
if (flags & SLAB_STORE_USER) {
- /* user store requires word alignment and
- * one word storage behind the end of the real
- * object.
+ /* user store requires one word storage behind the end of
+ * the real object.
*/
- align = BYTES_PER_WORD;
size += BYTES_PER_WORD;
}
#if FORCED_DEBUG && defined(CONFIG_DEBUG_PAGEALLOC)
-
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