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

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Message-ID: <98996e39-7d29-354c-9009-d4b1a1bbdeb0@kernel.dk>
Date:   Fri, 21 Sep 2018 09:00:31 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Christoph Hellwig <hch@....de>,
        Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     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>,
        Ming Lei <ming.lei@...hat.com>,
        Christoph Lameter <cl@...ux.com>
Subject: Re: block: DMA alignment of IO buffer allocated from slab

On 9/21/18 7:05 AM, Christoph Hellwig 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.
>>
>> 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..

Fully agree, it's such a fundamental property.

-- 
Jens Axboe