Re: [patch 2/2] slab: always consider arch mandated alignment

From: Manfred Spraul
Date: Wed Jul 26 2006 - 14:43:31 EST

Christoph Lameter wrote:

On Wed, 26 Jul 2006, Manfred Spraul wrote:

Signed-off-by: Manfred Spraul <manfred@xxxxxxxxxxxxxxxx>

Good bye to all those cacheline contentions that helped us find so many race conditions in the past if we switched on SLAB_DEBUG. I thought this was intentional?

Relax, align is nearly never set.

- kmalloc uses align==0, except if the architecture requests it (ARCH_KMALLOC_MINALIGN not 0)
- on my i386 system, the following users explicitely use align:
* the pmd structure (4096: hardware requirement)
* the pgd structure (32 bytes: hardware requirement)
* the task structure (16 byte. fxsave)
* sigqueue, pid: both request 4 byte alignment (based on __alignof__()). Doesn't affect debugging.

From the other mail:

Thus the patch is correct, it's a bug in the slab allocator. If HWCACHE_ALIGN
is set, then the allocator ignores align or ARCH_SLAB_MINALIGN.

But then Heiko does not want to set ARCH_SLAB_MINALIGN at all. This is not the issue we are discussing. In the DEBUG case he wants ARCH_KMALLOC_MINALIGN to be enforced even if ARCH_SLAB_MINALIGN is not set.

The kmalloc caches are allocated with HWCACHE_ALIGN+ARCH_KMALLOC_MINALIGN. The logic in kmem_cache_create didn't handle that case correctly.
On most architectures, ARCH_KMALLOC_MINALIGN is 0. Thus SLAB_DEBUG redzones everything.
On s390, ARCH_KMALLOC_MINALIGN is 8. This disables redzoning.


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