linux-kernel - Re: block: DMA alignment of IO buffer allocated from slab

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Date:   Mon, 24 Sep 2018 07:19:17 -0700
From:   Bart Van Assche <bvanassche@....org>
To:     Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Ming Lei <ming.lei@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     Christoph Hellwig <hch@....de>, Ming Lei <tom.leiming@...il.com>,
        linux-block <linux-block@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        "open list:XFS FILESYSTEM" <linux-xfs@...r.kernel.org>,
        Dave Chinner <dchinner@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Jens Axboe <axboe@...nel.dk>, Christoph Lameter <cl@...ux.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: block: DMA alignment of IO buffer allocated from slab

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@....de> 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.