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

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Date:	Wed, 26 Jul 2006 12:47:05 -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:

> Christoph Lameter wrote:
> 
> > A slab user is setting alignment in order to optimize performance not for
> > correctness. Most users that I know of can live with misalignments. If that
> > would not be the case then this code would never have worked.
> >  
> 
> Which users do you know that set align and that can live with misalignments?
> As I wrote, there are no such users in my (i386) kernel.

The users of SLAB_HWCACHE_ALIGN can live with that.

Systems running with slab debugging on must be very buggy at this point or 
we were very lucky:

The list is a bit strange:

>* the pmd structure (4096: hardware requirement)

It is already exempted from debug since the size is 4096.

>* the pgd structure (32 bytes: hardware requirement)

We were lucky on that one in the past? This should break.

>* the task structure (16 byte. fxsave)

Would only break if floating point is used I think.

>* sigqueue, pid: both request 4 byte alignment (based on __alignof__()). 
>Doesn't affect debugging.
So also not relevant.


We now want to say that SLAB_HWCACHE_ALIGN is only a suggestion to be 
disposed of if debug is on whereas an explicitly specified alignment must be enforced?

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