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

[Jens Axboe]

On 9/24/18 10:06 AM, Christopher Lameter wrote:
> 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.

The summary is that, no, kmalloc(N) is not N-1 aligned and nobody should
rely on that. On the block side, a few drivers set DMA alignment to
the sector size. Given that things seem to Just Work, even with XFS doing
kmalloc(512) and submitting IO with that, I think we can fairly safely
assume that most of those drivers are just being overly cautious and are
probably quite fine with 4/8 byte alignment.

The situation is making me a little uncomfortable, though. If we export
such a setting, we really should be honoring it...

-- 
Jens Axboe