Re: [BUG] slab debug vs. L1 alignement

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Fri, 2003-08-15 at 23:50, Manfred Spraul wrote:
> Ben wrote:
> 
> >Currently, when enabling slab debugging, we lose the property of
> >having the objects aligned on a cache line size.
> >  
> >
> Correct. Cache line alignment is advisory. Slab debugging is not the 
> only case that violates the alignment, for example 32-byte allocations 
> are not padded to the 128 byte cache line size of the Pentium 4 cpus. I 
> really doubt we want that.

Yes, I understand that, but that is wrong for GFP_DMA imho. Also, 
SLAB_MUST_HWCACHE_ALIGN just disables redzoning, which is not smart,
I'd rather allocate more and keep both redzoning and cache alignement,
that would help catch some of those subtle problems when a chip DMA
engine plays funny tricks

> Have you looked at pci_pool_{create,alloc,free,destroy}? The functions 
> were specifically written to provide aligned buffers for DMA operations. 
> Perhaps SCSI should use them?

Of course it should, but it doesn't yet. And other drivers haven't been
ported yet neither though that is slowly happening. Still, I think the
point that a GFP_DMA should be cache aligned still stands, don't you
think ?

Ben.