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

[Bart Van Assche]

On 9/24/18 2:46 AM, Andrey Ryabinin wrote:
> On 09/24/2018 01:42 AM, Ming Lei 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.
>
> This assumption is not correct. And it's not correct at least from the beginning of the
> git era, which is even before SLUB allocator appeared. With CONFIG_DEBUG_SLAB=y
> the same as with CONFIG_SLUB_DEBUG_ON=y kmalloc return 'unaligned' objects.
> The guaranteed arch-and-config-independent alignment of kmalloc() result is "sizeof(void*)".
>
> If objects has higher alignment requirement, the could be allocated via specifically created kmem_cache.

Hello Andrey,

The above confuses me. Can you explain to me why the following comment 
is present in include/linux/slab.h?

/*
 * kmalloc and friends return ARCH_KMALLOC_MINALIGN aligned
 * pointers. kmem_cache_alloc and friends return ARCH_SLAB_MINALIGN
 * aligned pointers.
 */

Thanks,

Bart.