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Subject: Re: block: DMA alignment of IO buffer allocated from slab
To:     Bart Van Assche <bvanassche@acm.org>,
        Ming Lei <ming.lei@redhat.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>
Cc:     Christoph Hellwig <hch@lst.de>, Ming Lei <tom.leiming@gmail.com>,
        linux-block <linux-block@vger.kernel.org>,
        linux-mm <linux-mm@kvack.org>,
        Linux FS Devel <linux-fsdevel@vger.kernel.org>,
        "open list:XFS FILESYSTEM" <linux-xfs@vger.kernel.org>,
        Dave Chinner <dchinner@redhat.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Jens Axboe <axboe@kernel.dk>, Christoph Lameter <cl@linux.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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From:   Andrey Ryabinin <aryabinin@virtuozzo.com>
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Date:   Mon, 24 Sep 2018 17:43:10 +0300
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On 09/24/2018 05:19 PM, Bart Van Assche wrote:
> 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@lst.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*)".


Correction sizeof(unsigned long long), so 8-byte alignment guarantee.

>>
>> 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.
>  */
> 

ARCH_KMALLOC_MINALIGN - guaranteed alignment of the kmalloc() result.
ARCH_SLAB_MINALIGN - guaranteed alignment of kmem_cache_alloc() result.

If the 'align' argument passed into kmem_cache_create() is bigger than ARCH_SLAB_MINALIGN
than kmem_cache_alloc() from that cache should return 'align'-aligned pointers.


> Thanks,
> 
> Bart.