[PATCH V5 0/6] loop: improve loop aio perf by IOCB_NOWAIT

[Ming Lei <ming.lei@redhat.com>]

Hello Jens,

This patchset improves loop aio perf by using IOCB_NOWAIT for avoiding to queue aio
command to workqueue context, meantime refactor lo_rw_aio() a bit.

In my test VM, loop disk perf becomes very close to perf of the backing block
device(nvme/mq virtio-scsi).

And Mikulas verified that this way can improve 12jobs sequential readwrite io by
~5X, and basically solve the reported problem together with loop MQ change.

https://lore.kernel.org/linux-block/a8e5c76a-231f-07d1-a394-847de930f638@redhat.com/

Zhaoyang Huang also mentioned it may fix their performance issue on Android
use case.

The loop MQ change will be posted as standalone patch, because it needs
UAPI change.

V5:
	- only try nowait in case that backing file supports it (Yu Kuai)
	- fix one lockdep assert (syzbot) 
	- improve comment log (Christoph)

V4:
	- rebase
	- re-organize and make it more readable 

V3:
	- add reviewed-by tag
	- rename variable & improve commit log & comment on 5/5(Christoph)

V2:
	- patch style fix & cleanup (Christoph)
	- fix randwrite perf regression on sparse backing file
	- drop MQ change

Ming Lei (6):
  loop: add helper lo_cmd_nr_bvec()
  loop: add helper lo_rw_aio_prep()
  loop: add lo_submit_rw_aio()
  loop: move command blkcg/memcg initialization into loop_queue_work
  loop: try to handle loop aio command via NOWAIT IO first
  loop: add hint for handling aio via IOCB_NOWAIT

 drivers/block/loop.c | 233 +++++++++++++++++++++++++++++++++++--------
 1 file changed, 194 insertions(+), 39 deletions(-)

-- 
2.47.0