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Subject: Re: block: DMA alignment of IO buffer allocated from slab
To:     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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 <20180920063129.GB12913@lst.de> <87h8ij0zot.fsf@vitty.brq.redhat.com>
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From:   Andrey Ryabinin <aryabinin@virtuozzo.com>
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Date:   Mon, 24 Sep 2018 12:46:27 +0300
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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*)".

If objects has higher alignment requirement, the could be allocated via specifically created kmem_cache.


> 
> Even some of buffer address is _not_ L1 cache size aligned, this way is
> totally broken wrt. DMA to/from this buffer.
> 
> So this issue has to be fixed in slab debug side.
> 

Well, this definitely would increase memory consumption. Many (probably most) of the kmalloc()
users doesn't need such alignment, why should they pay the cost?