linux-kernel - Re: [PATCH v1 0/6] no-copy bvec

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Message-ID: <933030f0-e428-18fd-4668-68db4f14b976@gmail.com>
Date:   Wed, 23 Dec 2020 12:52:59 +0000
From:   Pavel Begunkov <asml.silence@...il.com>
To:     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 22/12/2020 14:11, Christoph Hellwig wrote:
> On Tue, Dec 15, 2020 at 02:05:35PM +0000, Pavel Begunkov wrote:
>>> You may find clue from the following link:
>>>
>>> https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg2262077.html
>>
>> Thanks for the link!
>>
>> Al, you mentioned "Zero-length segments are not disallowed", do you have
>> a strong opinion on that? Apart from already diverged behaviour from the
>> block layer and getting in the way of this series, without it we'd also be
>> able to remove some extra ifs, e.g. in iterate_bvec()
> 
> I'd prefer not to support zero-length ITER_BVEC and catching them
> early, as the block layer can't deal with them either.  From a quick
> look at iter_file_splice_write it should be pretty trivial to fix there,
> although we'll need to audit other callers as well (even if I don't
> expect them to submit this degenerate case).

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.

Thanks for other nits, they will go into next version.

-- 
Pavel Begunkov