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

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Message-ID: <Pine.LNX.4.64.0607261153220.6896@schroedinger.engr.sgi.com>
Date:	Wed, 26 Jul 2006 11:59:57 -0700 (PDT)
From:	Christoph Lameter <clameter@....com>
To:	Manfred Spraul <manfred@...orfullife.com>
cc:	Pekka J Enberg <penberg@...helsinki.fi>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Andrew Morton <akpm@...l.org>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, Martin Schwidefsky <schwidefsky@...ibm.com>
Subject: Re: [patch 2/2] slab: always consider arch mandated alignment

On Wed, 26 Jul 2006, Manfred Spraul wrote:

> > Thus the patch is correct, it's a bug in the slab allocator. If
> > HWCACHE_ALIGN
> > > is set, then the allocator ignores align or ARCH_SLAB_MINALIGN.
> >  
> > 
> > But then Heiko does not want to set ARCH_SLAB_MINALIGN at all. This is not
> > the issue we are discussing. In the DEBUG case he wants
> > ARCH_KMALLOC_MINALIGN to be enforced even if ARCH_SLAB_MINALIGN is not set.
> >  
> The kmalloc caches are allocated with HWCACHE_ALIGN+ARCH_KMALLOC_MINALIGN. The
> logic in kmem_cache_create didn't handle that case correctly.
> On most architectures, ARCH_KMALLOC_MINALIGN is 0. Thus SLAB_DEBUG redzones
> everything.
> On s390, ARCH_KMALLOC_MINALIGN is 8. This disables redzoning.
> 
> Ok?

So Redzoning etc will now be diabled regardless even if  
ARCH_SLAB_MINALIGN is not set but another alignment is given to 
kmem_cache_alloc?

So we sacrifice the ability to worsen the performance of slabs by 
misaligning them for debugging purposes.
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