[PATCH v2 0/7] no-copy bvec

[Pavel Begunkov <asml.silence@gmail.com>]

Currently, when iomap and block direct IO gets a bvec based iterator
the bvec will be copied, with all other accounting that takes much
CPU time and causes additional allocation for larger bvecs. The
patchset makes it to reuse the passed in iter bvec.

[1,2] are forbidding zero-length bvec segments to not pile special
cases, [3] skip/fix PSI tracking to not iterate over bvecs extra
time.


nullblk completion_nsec=0 submit_queues=NR_CORES, no merges, no stats
fio/t/io_uring /dev/nullb0 -d 128 -s 32 -c 32 -p 0 -B 1 -F 1 -b BLOCK_SIZE

BLOCK_SIZE             512     4K      8K      16K     32K     64K
===================================================================
old (KIOPS)            1208    1208    1131    1039    863     699
new (KIOPS)            1222    1222    1170    1137    1083    982

Previously, Jens got before 10% difference for polling real HW and small
block sizes, but that was for an older version that had one
iov_iter_advance() less


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

since v1:
- don't allow zero-length bvec segments (Ming)
- don't add a BIO_WORKINGSET-less version of bio_add_page(), just clear
  the flag at the end and leave it for further cleanups (Christoph)
- commit message and comments rewording (Dave)
- other nits by Christoph

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

Pavel Begunkov (6):
  splice: don't generate zero-len segement bvecs
  bvec/iter: disallow zero-length segment bvecs
  block/psi: remove PSI annotations from direct IO
  iov_iter: optimise bvec iov_iter_advance()
  bio: add a helper calculating nr segments to alloc
  bio: don't copy bvec for direct IO

 Documentation/filesystems/porting.rst | 16 ++++++
 block/bio.c                           | 71 +++++++++++++--------------
 drivers/target/target_core_file.c     | 20 +++-----
 fs/block_dev.c                        |  7 +--
 fs/direct-io.c                        |  2 +
 fs/iomap/direct-io.c                  |  9 ++--
 fs/splice.c                           |  9 ++--
 include/linux/bio.h                   | 13 +++++
 lib/iov_iter.c                        | 21 +++++++-
 9 files changed, 103 insertions(+), 65 deletions(-)

-- 
2.24.0