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From: Dave Chinner <[email protected]>
To: Pavel Begunkov <[email protected]>
Cc: [email protected], Jens Axboe <[email protected]>,
	Christoph Hellwig <[email protected]>,
	Matthew Wilcox <[email protected]>,
	Ming Lei <[email protected]>,
	Johannes Weiner <[email protected]>,
	Alexander Viro <[email protected]>,
	"Darrick J . Wong" <[email protected]>,
	"Martin K . Petersen" <[email protected]>,
	Jonathan Corbet <[email protected]>,
	[email protected], [email protected],
	[email protected], [email protected],
	[email protected], [email protected],
	[email protected]
Subject: Re: [PATCH v1 6/6] block/iomap: don't copy bvec for direct IO
Date: Tue, 15 Dec 2020 12:09:21 +1100	[thread overview]
Message-ID: <[email protected]> (raw)
In-Reply-To: <498b34d746627e874740d8315b2924880c46dbc3.1607976425.git.asml.silence@gmail.com>

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 <[email protected]>
> Signed-off-by: Pavel Begunkov <[email protected]>
> ---
>  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
[email protected]

  reply	other threads:[~2020-12-15  1:10 UTC|newest]

Thread overview: 38+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-12-15  0:20 [PATCH v1 0/6] no-copy bvec Pavel Begunkov
2020-12-15  0:20 ` [PATCH v1 1/6] target/file: allocate the bvec array as part of struct target_core_file_cmd Pavel Begunkov
2020-12-15  0:20 ` [PATCH v1 2/6] iov_iter: optimise bvec iov_iter_advance() Pavel Begunkov
2020-12-15  9:37   ` David Laight
2020-12-15 11:23     ` Pavel Begunkov
2020-12-15 13:54       ` David Laight
2020-12-15 13:56         ` Pavel Begunkov
2020-12-22 14:03   ` Christoph Hellwig
2020-12-15  0:20 ` [PATCH v1 3/6] bio: deduplicate adding a page into bio Pavel Begunkov
2020-12-22 14:04   ` Christoph Hellwig
2020-12-15  0:20 ` [PATCH v1 4/6] block/psi: remove PSI annotations from direct IO Pavel Begunkov
2020-12-15  0:56   ` Dave Chinner
2020-12-15  1:03     ` Pavel Begunkov
2020-12-15  1:33       ` Dave Chinner
2020-12-15 11:41         ` Pavel Begunkov
2020-12-22 14:07   ` Christoph Hellwig
2020-12-15  0:20 ` [PATCH v1 5/6] bio: add a helper calculating nr segments to alloc Pavel Begunkov
2020-12-15  1:00   ` Dave Chinner
2020-12-15  1:07     ` Pavel Begunkov
2020-12-15  1:09     ` Dave Chinner
2020-12-22 14:07   ` Christoph Hellwig
2020-12-15  0:20 ` [PATCH v1 6/6] block/iomap: don't copy bvec for direct IO Pavel Begunkov
2020-12-15  1:09   ` Dave Chinner [this message]
2020-12-15  1:15     ` Pavel Begunkov
2020-12-22 14:15   ` Christoph Hellwig
2020-12-15  1:41 ` [PATCH v1 0/6] no-copy bvec Ming Lei
2020-12-15 11:14   ` Pavel Begunkov
2020-12-15 12:03     ` Ming Lei
2020-12-15 14:05       ` Pavel Begunkov
2020-12-22 14:11         ` Christoph Hellwig
2020-12-23 12:52           ` Pavel Begunkov
2020-12-23 15:51             ` Christoph Hellwig
2020-12-23 16:04               ` James Bottomley
2020-12-23 20:23                 ` Douglas Gilbert
2020-12-23 20:32                   ` Pavel Begunkov
2020-12-24  6:41                     ` Christoph Hellwig
2020-12-24 16:45                       ` Douglas Gilbert
2020-12-24 17:30                   ` James Bottomley

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