Re: [patch] slab: always follow arch requested alignments

[Christoph Lameter]

On Sat, 22 Jul 2006, Heiko Carstens wrote:

> Sorry, I should have mentioned it: on s390 (32 bit) we set
> #define ARCH_KMALLOC_MINALIGN 8.
> This is needed since our common I/O layer allocates data structures that need
> to have an eight byte alignment. Now, if I turn on DEBUG_SLAB, nothing works
> anymore, simply because the slab cache code ignores ARCH_KMALLOC_MINALIGN and
> uses an BYTES_PER_WORD alignment instead, which it shouldn't:
> 
> also from mm/slab.c :
> 
> #ifndef ARCH_KMALLOC_MINALIGN
> /*
>  * Enforce a minimum alignment for the kmalloc caches.
>  * Usually, the kmalloc caches are cache_line_size() aligned, except when
>  * DEBUG and FORCED_DEBUG are enabled, then they are BYTES_PER_WORD aligned.
>  * Some archs want to perform DMA into kmalloc caches and need a guaranteed
>  * alignment larger than BYTES_PER_WORD. ARCH_KMALLOC_MINALIGN allows that.
>  * Note that this flag disables some debug features.
>  */
> #define ARCH_KMALLOC_MINALIGN 0
> #endif
> 
> Since that didn't work I thought why not set ARCH_SLAB_MINALIGN to 8, since
> that would (according to the description) guarantee that _all_ caches would
> have an 8 byte alignment. But that didn't work too.

Why did that not work

See kmem_cache_create():
      /* 2) arch mandated alignment: disables debug if necessary */
        if (ralign < ARCH_SLAB_MINALIGN) {
                ralign = ARCH_SLAB_MINALIGN;
                if (ralign > BYTES_PER_WORD)
                        flags &= ~(SLAB_RED_ZONE | SLAB_STORE_USER);
        }

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