linux-kernel - Re: next-20130204 - bisected slab problem to "slab: Common constants for kmalloc boundaries"

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Date:	Tue, 5 Feb 2013 18:52:27 +0000
From:	Christoph Lameter <cl@...ux.com>
To:	Stephen Warren <swarren@...dotorg.org>
cc:	James Hogan <james.hogan@...tec.com>,
	linux-next <linux-next@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Pekka Enberg <penberg@...nel.org>,
	Matt Mackall <mpm@...enic.com>, linux-mm@...ck.org
Subject: Re: next-20130204 - bisected slab problem to "slab: Common constants
 for kmalloc boundaries"

On Tue, 5 Feb 2013, Stephen Warren wrote:

> > +/*
> > + * Some archs want to perform DMA into kmalloc caches and need a guaranteed
> > + * alignment larger than the alignment of a 64-bit integer.
> > + * Setting ARCH_KMALLOC_MINALIGN in arch headers allows that.
> > + */
> > +#if defined(ARCH_DMA_MINALIGN) && ARCH_DMA_MINALIGN > 8
> > +#define ARCH_KMALLOC_MINALIGN ARCH_DMA_MINALIGN
> > +#define KMALLOC_MIN_SIZE ARCH_DMA_MINALIGN
>
> I might be tempted to drop that #define of KMALLOC_MIN_SIZE ...

Initially I thought so too.
>
> > +#define KMALLOC_SHIFT_LOW ilog2(ARCH_DMA_MINALIGN)
> > +#else
> > +#define ARCH_KMALLOC_MINALIGN __alignof__(unsigned long long)
> > +#endif
>
> > +#ifndef KMALLOC_MIN_SIZE
> >  #define KMALLOC_MIN_SIZE (1 << KMALLOC_SHIFT_LOW)
> >  #endif
>
> ... and simply drop the ifdef around that #define instead.

That is going to be one hell of a macro expansion.

> That way, KMALLOC_MIN_SIZE is always defined in one place, and derived
> from KMALLOC_SHIFT_LOW; the logic will just set KMALLOC_SHIFT_LOW based
> on the various conditions. This seems a little safer to me; fewer
> conditions and less code to update if anything changes.

Yeah but we do an ilog2 and then reverse this back to the original number.

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