linux-kernel - Re: [PATCH v1 6/6] block/iomap: don't copy bvec for direct IO

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Date:   Tue, 15 Dec 2020 12:09:21 +1100
From:   Dave Chinner <david@...morbit.com>
To:     Pavel Begunkov <asml.silence@...il.com>
Cc:     linux-block@...r.kernel.org, Jens Axboe <axboe@...nel.dk>,
        Christoph Hellwig <hch@...radead.org>,
        Matthew Wilcox <willy@...radead.org>,
        Ming Lei <ming.lei@...hat.com>,
        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 6/6] block/iomap: don't copy bvec for direct IO

On Tue, Dec 15, 2020 at 12:20:25AM +0000, Pavel Begunkov wrote:
> The block layer spends quite a while in blkdev_direct_IO() to copy and
> initialise bio's bvec. However, if we've already got a bvec in the input
> iterator it might be reused in some cases, i.e. when new
> ITER_BVEC_FLAG_FIXED flag is set. Simple tests show considerable
> performance boost, and it also reduces memory footprint.
> 
> Suggested-by: Matthew Wilcox <willy@...radead.org>
> Signed-off-by: Pavel Begunkov <asml.silence@...il.com>
> ---
>  Documentation/filesystems/porting.rst |  9 ++++
>  block/bio.c                           | 64 +++++++++++----------------
>  include/linux/bio.h                   |  3 ++
>  3 files changed, 38 insertions(+), 38 deletions(-)

This doesn't touch iomap code, so the title of the patch seems
wrong...

> +For bvec based itererators bio_iov_iter_get_pages() now doesn't copy bvecs but
> +uses the one provided. Anyone issuing kiocb-I/O should ensure that the bvec and
> +page references stay until I/O has completed, i.e. until ->ki_complete() has
> +been called or returned with non -EIOCBQUEUED code.

This is hard to follow. Perhaps:

bio_iov_iter_get_pages() uses the bvecs  provided for bvec based
iterators rather than copying them. Hence anyone issuing kiocb based
IO needs to ensure the bvecs and pages stay referenced until the
submitted I/O is completed by a call to ->ki_complete() or returns
with an error other than -EIOCBQUEUED.

> diff --git a/include/linux/bio.h b/include/linux/bio.h
> index 2a9f3f0bbe0a..337f4280b639 100644
> --- a/include/linux/bio.h
> +++ b/include/linux/bio.h
> @@ -444,6 +444,9 @@ static inline void bio_wouldblock_error(struct bio *bio)
>  
>  static inline int bio_iov_vecs_to_alloc(struct iov_iter *iter, int max_segs)
>  {
> +	/* reuse iter->bvec */
> +	if (iov_iter_is_bvec(iter))
> +		return 0;
>  	return iov_iter_npages(iter, max_segs);

Ah, I'm a blind idiot... :/

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com