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

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Date:   Tue, 15 Dec 2020 00:20:19 +0000
From:   Pavel Begunkov <asml.silence@...il.com>
To:     linux-block@...r.kernel.org
Cc:     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: [PATCH v1 0/6] no-copy bvec

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.

bio_iov_iter_get_pages() is still does iov_iter_advance(), which is
not great, but neccessary for revert to work. It's desirable to have
a fast version of iov_iter_advance(i, i->count), so we may want to
hack something up for that. E.g. allow to not keep it consistent
in some cases when i->count==0. Also we can add a separate bio pool
without inlined bvec. Very easy to do and shrinks bios from 3 to 2
cachelines.

Also as suggested it removes BIO_WORKINGSET from direct paths: blkdev,
iomap, fs/direct-io. Even though the last one is not very important as
more filesystems are converted to iomap, but still looks hacky. Maybe,
as Johannes mentioned in another thread, moving it to the writeback
code (or other option) would be better in the end. Afterwards?

since RFC:
- add target_core_file patch by Christoph
- make no-copy default behaviour, remove iter flag
- iter_advance() instead of hacks to revert to work
- add bvec iter_advance() optimisation patch
- remove PSI annotations from direct IO (iomap, block and fs/direct)
- note in d/f/porting

Christoph Hellwig (1):
  target/file: allocate the bvec array as part of struct
    target_core_file_cmd

Pavel Begunkov (5):
  iov_iter: optimise bvec iov_iter_advance()
  bio: deduplicate adding a page into bio
  block/psi: remove PSI annotations from direct IO
  bio: add a helper calculating nr segments to alloc
  block/iomap: don't copy bvec for direct IO

 Documentation/filesystems/porting.rst |   9 +++
 block/bio.c                           | 103 ++++++++++++--------------
 drivers/target/target_core_file.c     |  20 ++---
 fs/block_dev.c                        |   7 +-
 fs/direct-io.c                        |   2 +
 fs/iomap/direct-io.c                  |   9 +--
 include/linux/bio.h                   |   9 +++
 lib/iov_iter.c                        |  19 +++++
 8 files changed, 102 insertions(+), 76 deletions(-)

-- 
2.24.0