linux-kernel - Re: [patch 4/6] slub: avoid compiling SLAB_CACHE

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Date:	Wed, 25 May 2011 16:34:48 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Christoph Lameter <cl@...ux.com>
cc:	Pekka Enberg <penberg@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [patch 4/6] slub: avoid compiling SLAB_CACHE_DMA without DMA
 support

On Wed, 25 May 2011, Christoph Lameter wrote:

> > If CONFIG_ZONE_DMA is disabled, SLAB_CACHE_DMA is a no-op.  Avoid
> > compiling support for it on such a configuration.
> 
> Setting SLAB_CACHE_DMA to zero in the !ZONE_DMA case could avoid some of
> the ifdeffery.
> 

That doesn't actually accomplish anything since all SLAB_CACHE_DMA does is 
set SLUB_DMA, which is already 0 in slub_def.h without CONFIG_ZONE_DMA:

	#ifdef CONFIG_ZONE_DMA
	#define SLUB_DMA __GFP_DMA  
	#else
	/* Disable DMA functionality */
	#define SLUB_DMA (__force gfp_t)0
	#endif

So doing the same for SLAB_CACHE_DMA wouldn't do anything.  The point of 
the patchset is to break the build if SLAB_CACHE_DMA is passed to 
kmem_cache_create() and the kernel is not compiled for DMA support.  That 
should result in a build breakage rather than silently not supporting DMA 
slab caches since it needs to be corrected at build.
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