[BUG] slab debug vs. L1 alignement

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

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.

Ben.