linux-kernel - Re: [patch] slab: always follow arch requested alignments

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Date:	Sat, 22 Jul 2006 18:26:07 +0200
From:	Heiko Carstens <heiko.carstens@...ibm.com>
To:	Christoph Lameter <clameter@....com>
Cc:	Andrew Morton <akpm@...l.org>, linux-kernel@...r.kernel.org,
	Pekka Enberg <penberg@...helsinki.fi>
Subject: Re: [patch] slab: always follow arch requested alignments

On Sat, Jul 22, 2006 at 07:50:00AM -0700, Christoph Lameter wrote:
> On Sat, 22 Jul 2006, Heiko Carstens wrote:
> 
> > In kmem_cache_create(): always check if BYTES_PER_WORD is less than
> > ARCH_SLAB_MINALIGN and disable debug options that would set the
> > alignment to BYTES_PER_WORD.
> 
> Why disable debug options?

Because if they are still enabled we would end up with an BYTES_PER_WORD
alignment (which is bad, see below).

> > This will make sure that all slab caches will have at least an
> > ARCH_SLAB_MINALIGN alignment.
> 
> You can specify alignment at cache creation. Why do we need all slabs to 
> be aligned?

Uhm, that's the meaning of ARCH_SLAB_MINALIGN, isn't it?

from mm/slab.c :

#ifndef ARCH_SLAB_MINALIGN
/*
 * Enforce a minimum alignment for all caches.
 * Intended for archs that get misalignment faults even for BYTES_PER_WORD
 * aligned buffers. Includes ARCH_KMALLOC_MINALIGN.
 * If possible: Do not enable this flag for CONFIG_DEBUG_SLAB, it disables
 * some debug features.
 */
#define ARCH_SLAB_MINALIGN 0
#endif

> > In addition make sure that a caller mandated align which is greater
> > than BYTES_PER_WORD also disables the same debug options.
> > This makes sure that ARCH_KMALLOC_MINALIGN also has an effect if
> > CONFIG_DEBUG_SLAB is set.
> 
> Is there a particular problem you are trying to address?

Sorry, I should have mentioned it: on s390 (32 bit) we set
#define ARCH_KMALLOC_MINALIGN 8.
This is needed since our common I/O layer allocates data structures that need
to have an eight byte alignment. Now, if I turn on DEBUG_SLAB, nothing works
anymore, simply because the slab cache code ignores ARCH_KMALLOC_MINALIGN and
uses an BYTES_PER_WORD alignment instead, which it shouldn't:

also from mm/slab.c :

#ifndef ARCH_KMALLOC_MINALIGN
/*
 * Enforce a minimum alignment for the kmalloc caches.
 * Usually, the kmalloc caches are cache_line_size() aligned, except when
 * DEBUG and FORCED_DEBUG are enabled, then they are BYTES_PER_WORD aligned.
 * Some archs want to perform DMA into kmalloc caches and need a guaranteed
 * alignment larger than BYTES_PER_WORD. ARCH_KMALLOC_MINALIGN allows that.
 * Note that this flag disables some debug features.
 */
#define ARCH_KMALLOC_MINALIGN 0
#endif

Since that didn't work I thought why not set ARCH_SLAB_MINALIGN to 8, since
that would (according to the description) guarantee that _all_ caches would
have an 8 byte alignment. But that didn't work too.

So the result is this patch, which makes DEBUG_SLAB work on s390. And actually
guarantees what the above descriptions imply (unless the patch is broken).
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