linux-kernel - Re: [BUG] SLOB breaks Crypto

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Date:	Wed, 19 May 2010 08:44:43 +0300
From:	Pekka Enberg <penberg@...helsinki.fi>
To:	David Miller <davem@...emloft.net>
Cc:	mpm@...enic.com, herbert@...dor.apana.org.au, ken@...elabs.ch,
	geert@...ux-m68k.org, michael-dev@...i-braun.de,
	linux-kernel@...r.kernel.org, linux-crypto@...r.kernel.org,
	anemo@....ocn.ne.jp
Subject: Re: [BUG] SLOB breaks Crypto

Hi David,

On Wed, May 19, 2010 at 12:20 AM, David Miller <davem@...emloft.net> wrote:
>> Why? It doesn't make much sense for SLOB, which tries to be as space
>> efficient as possible, as a default. If things break on sparc, it
>> really needs to set ARCH_KMALLOC_MINALIGN as slab default alignment is
>> not something you really want to depend on.
>
> I think it does make sense to expect that, whatever my architecture
> defines or does not define, I can expect the allocators to provide the
> same minimum alignment guarentee.  Otherwise it is no guarantee at all.

They're not a guarantee, the default values are just "oh, you don't
care about alignment, let me provide one for you".

> I'll add the define for sparc, but saying "sparc's fault" is bogus
> because I defined what was necessary to get SLAB/SLUB to provide the
> necessary alignment.  SLOB pays for choosing not to use the same
> calculations for minimum alignment as the other allocators, and
> therefore pays for being different in this regard.
>
> And in fact I do know that the ifdef'ery in SLAB/SLUB is derived from
> a change long ago that was specifically added to handle platforms like
> sparc.
>
> So one of two things should happen:
>
> 1) SLOB conforms to SLAB/SLUB in it's test
>
> 2) SLAB/SLUB conforms to SLOB in it's test
>
> And yes this is an either-or, you can't say they are both valid.

I don't agree with that. The default values are subject to change and
as pointed out by Paul, they have done so in the past. If you
architecture has alignment requirements for _correctness_ you
absolutely need to define ARCH_KMALLOC_MINALIGN.

                         Pekka
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