[PATCH v4 0/5] dm: empty flush optimization

[Yang Yang]

__send_empty_flush() sends empty flush bios to every target in the
dm_table. However, if the num_targets exceeds the number of block
devices in the dm_table's device list, it could lead to multiple
invocations of __send_duplicate_bios() for the same block device.
Typically, a single thread sending numerous empty flush bios to one
block device is redundant, as these bios are likely to be merged by the
flush state machine. In scenarios where num_targets significantly
outweighs the number of block devices, such behavior may result in a
noteworthy decrease in performance.

This is a real-world scenario that we have encountered:
1) Call fallocate(file_fd, 0, 0, SZ_8G)
2) Call ioctl(file_fd, FS_IOC_FIEMAP, fiemap). In situations of severe
file system fragmentation, fiemap->fm_mapped_extents may exceed 1000.
3) Create a dm-linear device based on fiemap->fm_extents
4) Create a snapshot-cow device based on the dm-linear device 

Perf diff of fio test:
  fio --group_reporting --name=benchmark --filename=/dev/mapper/example \
      --ioengine=sync --invalidate=1 --numjobs=16 --rw=randwrite \
      --blocksize=4k --size=2G --time_based --runtime=30 --fdatasync=1

Scenario one:
  for i in {0..1023}; do
    echo $((8000*$i)) 8000 linear /dev/sda2 $((16384*$i))
  done | sudo dmsetup create example

  Before: bw=857KiB/
  After:  bw=30.8MiB/s    +3580%

Scenario two:
  for i in {0..1023}; do
    if [[ $i -gt 511 ]]; then
      echo $((8000*$i)) 8000 linear /dev/nvme0n1p6 $((16384*$i))
    else
      echo $((8000*$i)) 8000 linear /dev/sda2 $((16384*$i))
    fi
  done | sudo dmsetup create example

  Before: bw=1470KiB/
  After:  bw=33.9MiB/s    +2261%

Any comments are welcome!

V4:
-- Fix the build warning "no semicolon at end of struct or union."

V3:
-- Focus on targets with num_flush_bios equal to 1 to simplify the code
-- Use t->devices_lock to protect the dm_table's device list

V2:
-- Split into smaller pieces that are easier to review
-- Add flush_pass_around, suggested by Mikulas Patocka
-- Handling different target types separately

Yang Yang (5):
  dm: introduce flush_pass_around flag
  dm: add __send_empty_flush_bios() helper
  dm: support retrieving struct dm_target from struct dm_dev
  dm: Avoid sending redundant empty flush bios to the same block device
  dm linear: enable flush optimization function

 drivers/md/dm-core.h          |  3 +++
 drivers/md/dm-ioctl.c         |  4 ++++
 drivers/md/dm-linear.c        |  1 +
 drivers/md/dm-table.c         | 19 +++++++++++++++++++
 drivers/md/dm.c               | 34 +++++++++++++++++++++++++---------
 include/linux/device-mapper.h |  6 ++++++
 6 files changed, 58 insertions(+), 9 deletions(-)

-- 
2.34.1