Re: [BUG] slab debug vs. L1 alignement

[Philippe Elie]

Benjamin Herrenschmidt wrote:
> Currently, when enabling slab debugging, we lose the property of
> having the objects aligned on a cache line size.
>
> This is, imho, an error, especially if GFP_DMA is passed. Such an
> object _must_ be cache alined (and it's size rounded to a multiple
> of the cache line size).
>
> There is a simple performance reason on cache coherent archs, but
> there's also the fact that it will just _not_ work properly on
> non cache-coherent archs. Actually, I also have to deal with some
> old machines who have a SCSI controller who has a problem accessing
> buffers that aren't aligned on a cache line size boundary.
>
> This is typically causing me trouble in various parts of SCSI which
> abuses kmalloc(512, GFP_KERNEL | GFP_DMA) for buffers passed to some
> SCSI commands, typically "utility" commands used to read a disk
> capacity, read read/write protect flags, some sense buffers, etc...
>
> While I know SCSI shall use the consistent alloc things, it has not
> been fully fixed yet and kmalloc with GFP_DMA is still valid despite
> not beeing efficient, so we should make sure in this case, the returned
> buffer is really suitable for DMA, that is cache aligned.

Attached untested patch should fix it (vs 2.6.0-test1), I've no
idea if it's acceptable.

regards
Philippe Elie
--- mm/slab.c~	2003-07-14 03:36:48.000000000 +0000
+++ mm/slab.c	2003-08-15 18:40:14.000000000 +0000
@@ -682,7 +682,7 @@
 
 		sizes->cs_dmacachep = kmem_cache_create(
 			names->name_dma, sizes->cs_size,
-			0, SLAB_CACHE_DMA|SLAB_HWCACHE_ALIGN, NULL, NULL);
+			0, SLAB_CACHE_DMA|SLAB_MUST_HWCACHE_ALIGN, NULL, NULL);
 		if (!sizes->cs_dmacachep)
 			BUG();