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

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Date:	Wed, 26 Jul 2006 09:25:08 -0700 (PDT)
From:	Christoph Lameter <clameter@....com>
To:	Pekka Enberg <penberg@...helsinki.fi>
cc:	Heiko Carstens <heiko.carstens@...ibm.com>,
	Andrew Morton <akpm@...l.org>, linux-kernel@...r.kernel.org,
	Manfred Spraul <manfred@...orfullife.com>
Subject: Re: Re: [patch] slab: always follow arch requested alignments

On Wed, 26 Jul 2006, Pekka Enberg wrote:

> On 7/26/06, Christoph Lameter <clameter@....com> wrote:
> > We intentionally discard the caller mandated alignment for debugging
> > purposes.
> 
> Disagreed. The caller mandated alignment is not a hint. It is the
> required minimum alignment for objects.

This has been with the slab for a long time. Lots of alignments are now
ignored for the debugging case without a problem. Manfred intentionally
put that in. Alignments passed to kmem_cache_create are there for
performance reasons and not because unaligned object break the arch code.

> On 7/26/06, Christoph Lameter <clameter@....com> wrote:
> > And it changes the basic way that slab debugging works.
> 
> Look at kmem_cache_create, we turn off debugging for both caller and
> architecture mandated alignments already and the only reason we are
> not doing it for Heiko is because the architecture recommended default
> alignment is so large.

We discard alignment for FORCED_DEBUG unless

1. object size > 4096

2. If the object size would increase unreasonably.

I simply added another case. That preseves the discarding of alignment for 
the FORCED_DEBUG case but allows an override if operations would be 
impossible for not correctly aligned objects for certain caches (like in 
the S/390 case).
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