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

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Message-ID: <20180920063226.GC12913@lst.de>
Date:   Thu, 20 Sep 2018 08:32:26 +0200
From:   Christoph Hellwig <hch@....de>
To:     Ming Lei <ming.lei@...hat.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        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>,
        Christoph Hellwig <hch@....de>, Jens Axboe <axboe@...nel.dk>
Subject: Re: block: DMA alignment of IO buffer allocated from slab

On Thu, Sep 20, 2018 at 09:28:37AM +0800, Ming Lei wrote:
> > has e.g. PAGE_SIZE alignment requirement (this would likely imply that
> > it's sector size is also not 512 I guess)?
> 
> Yeah, that can be true if one controller has 4k-byte sector size, also
> its DMA alignment is 4K. But there shouldn't be cases in which the two
> doesn't match.

The general block storage worlds is that devices always need to have
an alignment requirement <= minimum LBAs size.  If they don't they'll
need to bounce buffer (in the driver!).