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

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Message-ID: <Pine.LNX.4.64.0607230558560.15651@schroedinger.engr.sgi.com>
Date:	Sun, 23 Jul 2006 06:03:09 -0700 (PDT)
From:	Christoph Lameter <clameter@....com>
To:	Heiko Carstens <heiko.carstens@...ibm.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 Sun, 23 Jul 2006, Heiko Carstens wrote:

> > 
> > See kmem_cache_create():
> >       /* 2) arch mandated alignment: disables debug if necessary */
> >         if (ralign < ARCH_SLAB_MINALIGN) {
> >                 ralign = ARCH_SLAB_MINALIGN;
> >                 if (ralign > BYTES_PER_WORD)
> >                         flags &= ~(SLAB_RED_ZONE | SLAB_STORE_USER);
> >         }
> 
> That is because if kmem_cache_create gets called with SLAB_HWCACHE_ALIGN set
> in flags then ralign will be greater or equal to ARCH_SLAB_MINALIGN:
> 
>         /* 1) arch recommendation: can be overridden for debug */ 
>         if (flags & SLAB_HWCACHE_ALIGN) { 
> 	        [...]
>                 ralign = cache_line_size(); 
> 	        [...]

Ok. Then you do not have a problem because ralign is greater than
ARCH_SLAB_MINALIGN.
 
> Therefore the test above will be passed and SLAB_RED_ZONE and SLAB_STORE_USER
> will stay in flags.
> cache_line_size() will return 256 on s390.

Looks as if you would have the correct alignment then. I still do not 
understand where the problem is since you want to align on an 8 byte 
boundary.


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