Hi David,
On Wed, May 19, 2010 at 12:20 AM, David Miller <davem@xxxxxxxxxxxxx> 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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