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

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Date:   Mon, 24 Sep 2018 09:19:44 -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 Mon, 2018-09-24 at 19:07 +0300, Andrey Ryabinin wrote:
> On 09/24/2018 06:58 PM, Bart Van Assche wrote:
> > On Mon, 2018-09-24 at 18:52 +0300, Andrey Ryabinin wrote:
> > > Yes, with CONFIG_DEBUG_SLAB=y, CONFIG_SLUB_DEBUG_ON=y kmalloc() guarantees
> > > that result is aligned on ARCH_KMALLOC_MINALIGN boundary.
> > 
> > Had you noticed that Vitaly Kuznetsov showed that this is not the case? See
> > also https://lore.kernel.org/lkml/87h8ij0zot.fsf@vitty.brq.redhat.com/.
> 
> I'm not following. On x86-64 ARCH_KMALLOC_MINALIGN is 8, all pointers that
> Vitaly Kuznetsov showed are 8-byte aligned.

Hi Andrey,

That means that two buffers allocated with kmalloc() may share a cache line on
x86-64. Since it is allowed to use a buffer allocated by kmalloc() for DMA, can
this lead to data corruption, e.g. if the CPU writes into one buffer allocated
with kmalloc() and a device performs a DMA write to another kmalloc() buffer and
both write operations affect the same cache line?

Thanks,

Bart.