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

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Date:   Mon, 24 Sep 2018 16:47:53 +0000
From:   Christopher Lameter <cl@...ux.com>
To:     Bart Van Assche <bvanassche@....org>
cc:     Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Ming Lei <ming.lei@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        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>,
        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, 24 Sep 2018, Bart Van Assche wrote:

> 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?

The devices writes to the cacheline through the processor which serializes
access appropriately.

The DMA device cannot write directly to memory after all on current Intel
processors. Other architectures have bus protocols that prevent situations
like that.