no-copy bvec

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

since v2:
- add a comment in 1/7 (Christoph)
- add a note about 0-len bvecs in biovecs.rst (Matthew)

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/block/biovecs.rst       |  2 +
 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 +++++++-
 10 files changed, 106 insertions(+), 64 deletions(-)

2.24.0