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Date:   Thu, 24 Dec 2020 06:41:19 +0000
From:   Christoph Hellwig <hch@...radead.org>
To:     Pavel Begunkov <asml.silence@...il.com>
Cc:     dgilbert@...erlog.com,
        James Bottomley <James.Bottomley@...senpartnership.com>,
        Christoph Hellwig <hch@...radead.org>,
        Ming Lei <ming.lei@...hat.com>, linux-block@...r.kernel.org,
        Jens Axboe <axboe@...nel.dk>,
        Matthew Wilcox <willy@...radead.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        "Darrick J . Wong" <darrick.wong@...cle.com>,
        "Martin K . Petersen" <martin.petersen@...cle.com>,
        Jonathan Corbet <corbet@....net>, linux-xfs@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, io-uring@...r.kernel.org,
        linux-kernel@...r.kernel.org, target-devel@...r.kernel.org,
        linux-scsi@...r.kernel.org, linux-doc@...r.kernel.org
Subject: Re: [PATCH v1 0/6] no-copy bvec

On Wed, Dec 23, 2020 at 08:32:45PM +0000, Pavel Begunkov wrote:
> On 23/12/2020 20:23, Douglas Gilbert wrote:
> > On 2020-12-23 11:04 a.m., James Bottomley wrote:
> >> On Wed, 2020-12-23 at 15:51 +0000, Christoph Hellwig wrote:
> >>> On Wed, Dec 23, 2020 at 12:52:59PM +0000, Pavel Begunkov wrote:
> >>>> Can scatterlist have 0-len entries? Those are directly translated
> >>>> into bvecs, e.g. in nvme/target/io-cmd-file.c and
> >>>> target/target_core_file.c. I've audited most of others by this
> >>>> moment, they're fine.
> >>>
> >>> For block layer SGLs we should never see them, and for nvme neither.
> >>> I think the same is true for the SCSI target code, but please double
> >>> check.
> >>
> >> Right, no-one ever wants to see a 0-len scatter list entry.?? The reason
> >> is that every driver uses the sgl to program the device DMA engine in
> >> the way NVME does.?? a 0 length sgl would be a dangerous corner case:
> >> some DMA engines would ignore it and others would go haywire, so if we
> >> ever let a 0 length list down into the driver, they'd have to
> >> understand the corner case behaviour of their DMA engine and filter it
> >> accordingly, which is why we disallow them in the upper levels, since
> >> they're effective nops anyway.
> > 
> > When using scatter gather lists at the far end (i.e. on the storage device)
> > the T10 examples (WRITE SCATTERED and POPULATE TOKEN in SBC-4) explicitly
> > allow the "number of logical blocks" in their sgl_s to be zero and state
> > that it is _not_ to be considered an error.
> 
> It's fine for my case unless it leaks them out of device driver to the
> net/block layer/etc. Is it?

None of the SCSI Command mentions above are supported by Linux,
nevermind mapped to struct scatterlist.

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