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    SubjectRe: [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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