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Date:   Wed, 23 Dec 2020 20:32:45 +0000
From:   Pavel Begunkov <asml.silence@...il.com>
To:     dgilbert@...erlog.com,
        James Bottomley <James.Bottomley@...senPartnership.com>,
        Christoph Hellwig <hch@...radead.org>
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 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?

-- 
Pavel Begunkov

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