linux-kernel - Re: [patch 2/2] slab: always consider arch mandated alignment

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Date:	Wed, 26 Jul 2006 14:16:06 +0300 (EEST)
From:	Pekka J Enberg <penberg@...Helsinki.FI>
To:	Heiko Carstens <heiko.carstens@...ibm.com>
cc:	Christoph Lameter <clameter@....com>,
	Andrew Morton <akpm@...l.org>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, Martin Schwidefsky <schwidefsky@...ibm.com>,
	manfred@...orfullife.com
Subject: Re: [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@...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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