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

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Message-ID: <103235c1-e7d0-0b55-65d0-013d1a09304e@gmail.com>
Date:   Tue, 15 Dec 2020 11:14:20 +0000
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Ming Lei <ming.lei@...hat.com>
Cc:     linux-block@...r.kernel.org, Jens Axboe <axboe@...nel.dk>,
        Christoph Hellwig <hch@...radead.org>,
        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 15/12/2020 01:41, Ming Lei wrote:
> On Tue, Dec 15, 2020 at 12:20:19AM +0000, Pavel Begunkov wrote:
>> Instead of creating a full copy of iter->bvec into bio in direct I/O,
>> the patchset makes use of the one provided. It changes semantics and
>> obliges users of asynchronous kiocb to track bvec lifetime, and [1/6]
>> converts the only place that doesn't.
> 
> Just think of one corner case: iov_iter(BVEC) may pass bvec table with zero
> length bvec, which may not be supported by block layer or driver, so
> this patchset has to address this case first.

The easiest for me would be to fallback to copy if there are zero bvecs,
e.g. finding such during iov_iter_alignment(), but do we know from where
zero bvecs can came? As it's internals we may want to forbid them if
there is not too much hassle.

> 
> Please see 7e24969022cb ("block: allow for_each_bvec to support zero len bvec").

-- 
Pavel Begunkov