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

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Message-ID: <20180925044421.GA11163@bombadil.infradead.org>
Date:   Mon, 24 Sep 2018 21:44:21 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Bart Van Assche <bvanassche@....org>
Cc:     Ming Lei <ming.lei@...hat.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.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>, 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, Sep 24, 2018 at 09:10:43PM -0700, Bart Van Assche wrote:
> On 9/24/18 8:28 PM, Matthew Wilcox wrote:
> > [ ... ] Because if we have to
> > round all allocations below 64 bytes up to 64 bytes, [ ... ]
> Have you noticed that in another e-mail in this thread it has been explained
> why it is not necessary on x86 to align buffers allocated by kmalloc() on a
> 64-byte boundary even if these buffers are used for DMA?

Oh, so drivers which do this only break on !x86.  Yes, that'll work
out great.