[RFC PATCH v2 0/9] simplify block layer based on immutable biovecs

[Dongsu Park <dongsu.park@profitbricks.com>]

This is the second attempt of simplifying block layer based on immutable
biovecs. Immutable biovecs, implemented by Kent Overstreet, have been
available in mainline since v3.14. Its original goal was actually making
generic_make_request() accept arbitrarily sized bios, and pushing the
splitting down to the drivers or wherever it's required. See also
discussions in the past, [1] [2] [3] [4].
  
This will bring not only performance improvements, but also a great amount
of reduction in code complexity all over the block layer. Performance gain
is possible due to the fact that bio_add_page() does not have to check
unnecesary conditions such as queue limits or if biovecs are mergeable.
Those will be delegated to the driver level. Kent already said that he
actually benchmarked the impact of this with fio on a micron p320h, which
showed definitely a positive impact.
  
Moreover, this patchset also allows a lot of code to be deleted, mainly
because of removal of merge_bvec_fn() callbacks. We have been aware that
it has been always a delicate issue for stacking block drivers (e.g. md
and bcache) to handle merging bio consistently. This simplication will
help every individual block driver avoid having such an issue.
  
- Patch 01/09 allows generic_make_request handle arbitrarily sized bios,
  by making make_request functions call blk_queue_split().
- Patch 02/09 simplifies __bio_add_page() to avoid calling ->merge_bvec_fn().
- Patch 03/09 allows queue_bounce to handle bios with > BIO_MAX_PAGES
- Patch 04/09 gets rid of workarounds in bcache.
- Patch 05/09 removes unnecessary biovec merging parts in btrfs
- Patch 06/09 removes unnecessary biovec merging parts in MD-RAID5.
- Patch 07/09 removes ->merge_bvec_fn() completely, which affects a lot of
  block drivers, such as Ceph RBD, DRBD, device mapper, MD, etc.
- Patch 08/09 makes filesystems use helper bio_add_page().
- Patch 09/09 updates document about biovecs.

Patches are against 3.19-rc4. These are also available in my git repo at:

  https://github.com/dongsupark/linux.git block-generic-req

It's recommended to apply this patchset on top of its preparation patchset
i.e. "preparation for block layer simplification". [5]
This patchset is in turn also a prerequisite of other consecutive patchsets,
e.g. multipage biovecs, rewriting plugging, or rewriting direct-IO, which
is yet to-be-done. This patchset should not bring any regression to
end-users. I already tested it with xfstests multiple times. On the other
hand, the multipage biovecs part is currently in heavy development, with
help of Kent and Ming Lin. Those experimental patches are also available
on other branches on my git tree. Once they are done, I'm also going to
post them to get reviews.

Comments are welcome.
Dongsu

Changes in v2:
- split up preparation patches into a separate series
- remove a patch "block: simplify issueing discard, write_same, zeroout",
  as suggested by Christoph Hellwig.
- move a patch "btrfs: make use of immutable biovecs" to the upcoming series.
- minor change in ps3vram suggested by Geoff Levand
- make bio_add_page() warn once on a cloned bio.
- add more comments and commit messages to patch 02 "block: simplify
  bio_add_page()"

[1] https://lkml.org/lkml/2014/11/23/263
[2] https://lkml.org/lkml/2013/11/25/732
[3] https://lkml.org/lkml/2014/2/26/618
[4] https://lkml.org/lkml/2014/12/22/128
[5] https://lkml.org/lkml/2015/1/12/255

Dongsu Park (1):
  Documentation: update notes in biovecs about arbitrarily sized bios

Kent Overstreet (8):
  block: make generic_make_request handle arbitrarily sized bios
  block: simplify bio_add_page()
  block: allow __blk_queue_bounce() to handle bios larger than
    BIO_MAX_PAGES
  bcache: clean up hacks around bio_split_pool
  btrfs: remove bio splitting and merge_bvec_fn() calls
  md/raid5: get rid of bio_fits_rdev()
  block: kill merge_bvec_fn() completely
  fs: use helper bio_add_page() instead of open coding on bi_io_vec

 Documentation/block/biovecs.txt             |  10 +-
 block/bio.c                                 | 135 +++++++++++----------------
 block/blk-core.c                            |  19 ++--
 block/blk-merge.c                           | 140 ++++++++++++++++++++++++++--
 block/blk-mq.c                              |   2 +
 block/blk-settings.c                        |  22 -----
 block/bounce.c                              |  60 ++++++++++--
 drivers/block/drbd/drbd_int.h               |   1 -
 drivers/block/drbd/drbd_main.c              |   1 -
 drivers/block/drbd/drbd_req.c               |  37 +-------
 drivers/block/pktcdvd.c                     |  27 +-----
 drivers/block/ps3vram.c                     |   2 +
 drivers/block/rbd.c                         |  47 ----------
 drivers/block/rsxx/dev.c                    |   2 +
 drivers/block/umem.c                        |   2 +
 drivers/block/zram/zram_drv.c               |   2 +
 drivers/md/bcache/bcache.h                  |  18 ----
 drivers/md/bcache/io.c                      | 100 +-------------------
 drivers/md/bcache/journal.c                 |   4 +-
 drivers/md/bcache/request.c                 |  16 ++--
 drivers/md/bcache/super.c                   |  32 +------
 drivers/md/bcache/util.h                    |   5 +-
 drivers/md/bcache/writeback.c               |   4 +-
 drivers/md/dm-cache-target.c                |  21 -----
 drivers/md/dm-crypt.c                       |  16 ----
 drivers/md/dm-era-target.c                  |  15 ---
 drivers/md/dm-flakey.c                      |  16 ----
 drivers/md/dm-linear.c                      |  16 ----
 drivers/md/dm-snap.c                        |  15 ---
 drivers/md/dm-stripe.c                      |  21 -----
 drivers/md/dm-table.c                       |   8 --
 drivers/md/dm-thin.c                        |  31 ------
 drivers/md/dm-verity.c                      |  16 ----
 drivers/md/dm.c                             | 122 +-----------------------
 drivers/md/dm.h                             |   2 -
 drivers/md/linear.c                         |  46 ---------
 drivers/md/md.c                             |   4 +-
 drivers/md/md.h                             |   8 --
 drivers/md/multipath.c                      |  21 -----
 drivers/md/raid0.c                          |  57 -----------
 drivers/md/raid0.h                          |   2 -
 drivers/md/raid1.c                          |  59 +-----------
 drivers/md/raid10.c                         | 122 +-----------------------
 drivers/md/raid5.c                          |  51 +---------
 drivers/s390/block/dcssblk.c                |   2 +
 drivers/s390/block/xpram.c                  |   2 +
 drivers/staging/lustre/lustre/llite/lloop.c |   2 +
 fs/btrfs/volumes.c                          |  73 ---------------
 fs/buffer.c                                 |   7 +-
 fs/jfs/jfs_logmgr.c                         |  14 +--
 include/linux/blkdev.h                      |  13 +--
 include/linux/device-mapper.h               |   4 -
 mm/page_io.c                                |   8 +-
 53 files changed, 318 insertions(+), 1164 deletions(-)

-- 
2.1.0