Re: block: DMA alignment of IO buffer allocated from slab

[Christopher Lameter]

On Fri, 21 Sep 2018, Christoph Hellwig wrote:

> On Fri, Sep 21, 2018 at 03:04:18PM +0200, Vitaly Kuznetsov wrote:
> > Christoph Hellwig <hch@xxxxxx> writes:
> >
> > > On Wed, Sep 19, 2018 at 05:15:43PM +0800, Ming Lei wrote:
> > >> 1) does kmalloc-N slab guarantee to return N-byte aligned buffer?  If
> > >> yes, is it a stable rule?
> > >
> > > This is the assumption in a lot of the kernel, so I think if somethings
> > > breaks this we are in a lot of pain.
> >
> > It seems that SLUB debug breaks this assumption. Kernel built with
> >
> > CONFIG_SLUB_DEBUG=y
> > CONFIG_SLUB=y
> > CONFIG_SLUB_DEBUG_ON=y
>
> Looks like we should fix SLUB debug then..

Nope. We need to not make unwarranted assumptions. Alignment is guaranteed
to ARCH_KMALLOC_MINALIGN for kmalloc requests. Fantasizing about
alighments and guessing from alignments that result on a particular
hardware and slab configuration that these are general does not work.