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Subject: Re: [PATCH v1 0/6] no-copy bvec
From:   James Bottomley <James.Bottomley@HansenPartnership.com>
To:     Christoph Hellwig <hch@infradead.org>,
        Pavel Begunkov <asml.silence@gmail.com>
Cc:     Ming Lei <ming.lei@redhat.com>, linux-block@vger.kernel.org,
        Jens Axboe <axboe@kernel.dk>,
        Matthew Wilcox <willy@infradead.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        "Darrick J . Wong" <darrick.wong@oracle.com>,
        "Martin K . Petersen" <martin.petersen@oracle.com>,
        Jonathan Corbet <corbet@lwn.net>, linux-xfs@vger.kernel.org,
        linux-fsdevel@vger.kernel.org, io-uring@vger.kernel.org,
        linux-kernel@vger.kernel.org, target-devel@vger.kernel.org,
        linux-scsi@vger.kernel.org, linux-doc@vger.kernel.org
Date:   Wed, 23 Dec 2020 08:04:22 -0800
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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.

James