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Date:   Wed, 23 Dec 2020 15:23:57 -0500
From:   Douglas Gilbert <dgilbert@...erlog.com>
To:     James Bottomley <James.Bottomley@...senPartnership.com>,
        Christoph Hellwig <hch@...radead.org>,
        Pavel Begunkov <asml.silence@...il.com>
Cc:     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 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.

Doug Gilbert

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