[PATCH v2 0/2] block: avoid redundant flushes for O_DSYNC direct writes

This series removes redundant cache flushes on the O_DIRECT | O_DSYNC
write path to a block device, following Christoph's review of v1.

  Patch 1 skips generic_write_sync() when the direct write was already
  made durable by REQ_FUA.

  Patch 2 stops setting REQ_FUA on synchronous O_DSYNC direct writes to
  devices without FUA support, relying on generic_write_sync() instead
  and avoiding per-bio POSTFLUSH emulation.  Asynchronous writes keep
  REQ_FUA, since they cannot fall back to generic_write_sync().

Changes since v1
[https://lore.kernel.org/linux-block/[email protected]/]:

  - Split into two patches per Christoph's suggestion.
  - Patch 1 tracks need_sync (default false) instead of dio_fua_done.
  - Added Patch 2 for the no-FUA case; asynchronous submitters stay at
    parity with mainline.

Tested on Linux 7.2.0-rc7 with a Seagate ST20000NM007D (fua=1), 4 KiB
O_DSYNC direct writes, mainline vs this series:

  Sequential: 119.7 -> 7497.0 IOPS
  Random:     156.1 ->  666.4 IOPS

On a loop device without FUA there was no regression, and async
O_DSYNC direct writes were verified durable with fio verify=crc32c
(262144 IOs, no errors).

Zhenxian Ma (2):
  block: skip redundant flush for O_DSYNC direct writes
  block: only use REQ_FUA for direct writes if the device supports it

 block/fops.c | 40 +++++++++++++++++++++++++++++++++-------
 1 file changed, 33 insertions(+), 7 deletions(-)

-- 
2.43.5