Re: [PATCH] slob: push the min alignment to long long

[Pekka Enberg <penberg@kernel.org>]

On Thu, Jun 16, 2011 at 1:53 AM, Matt Mackall <mpm@selenic.com> wrote:
>> Blink... because the compiler doesn't provide a portable way to
>> do this, right? :-)
>
> Because I, on x86, cannot deduce the alignment requirements of, say,
> CRIS without doing significant research. So answering a question like
> "are there any architectures where assumption X fails" is obnoxiously
> hard, rather than being a grep.
>
> I also don't think it's a given there's a portable way to deduce the
> alignment requirements due to the existence of arch-specific quirks. If
> an arch wants to kmalloc its weird crypto or SIMD context and those want
> 128-bit alignment, we're not going to want to embed that knowledge in
> the generic code, but instead tweak an arch define.
>
> Also note that not having generic defaults forces each new architectures
> to (nominally) examine each assumption rather than discover they
> inherited an incorrect default somewhere down the road.

I don't agree. I think we should either provide defaults that work for
everyone and let architectures override them (which AFAICT Christoph's
patch does) or we flat out #error if architectures don't specify
alignment requirements. The current solution seems to be the worst one
from practical point of view.

This doesn't seem to be a *regression* per se so I'll queue
Christoph's patch for 3.1 and mark it for 3.0-stable.

                            Pekka

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