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Date: Mon, 24 Sep 2018 16:06:28 +0000
From: Christopher Lameter <cl@linux.com>
Subject: Re: block: DMA alignment of IO buffer allocated from slab
In-Reply-To: <20180921130504.GA22551@lst.de>
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References: <CACVXFVOBq3L_EjSTCoiqUL1PH=HMR5EuNNQV0hNndFpGxmUK6g@mail.gmail.com> <20180920063129.GB12913@lst.de> <87h8ij0zot.fsf@vitty.brq.redhat.com> <20180921130504.GA22551@lst.de>
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To: Christoph Hellwig <hch@lst.de>
Cc: Vitaly Kuznetsov <vkuznets@redhat.com>, 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>, Ming Lei <ming.lei@redhat.com>

On Fri, 21 Sep 2018, Christoph Hellwig wrote:

> On Fri, Sep 21, 2018 at 03:04:18PM +0200, Vitaly Kuznetsov wrote:
> > Christoph Hellwig <hch@lst.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..

Nope. We need to not make unwarranted assumptions. Alignment is guaranteed
to ARCH_KMALLOC_MINALIGN for kmalloc requests. Fantasizing about
alighments and guessing from alignments that result on a particular
hardware and slab configuration that these are general does not work.