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Date: Mon, 24 Sep 2018 21:44:21 -0700
From: Matthew Wilcox <willy@infradead.org>
Subject: Re: block: DMA alignment of IO buffer allocated from slab
Message-ID: <20180925044421.GA11163@bombadil.infradead.org>
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To: Bart Van Assche <bvanassche@acm.org>
Cc: Ming Lei <ming.lei@redhat.com>, Andrey Ryabinin <aryabinin@virtuozzo.com>, Vitaly Kuznetsov <vkuznets@redhat.com>, 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>

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.