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

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Message-ID: <010001660c54fb65-b9d3a770-6678-40d0-8088-4db20af32280-000000@email.amazonses.com>
Date:   Mon, 24 Sep 2018 16:06:28 +0000
From:   Christopher Lameter <cl@...ux.com>
To:     Christoph Hellwig <hch@....de>
cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        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>, Ming Lei <ming.lei@...hat.com>
Subject: Re: block: DMA alignment of IO buffer allocated from slab

On Fri, 21 Sep 2018, Christoph Hellwig wrote:

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