block: loop: improve loop with AIO

Hi Guys,

There are about 3 advantages to use direct I/O and AIO on
read/write loop's backing file:

1) double cache can be avoided, then memory usage gets
decreased a lot

2) not like user space direct I/O, there isn't cost of
pinning pages

3) avoid context switch for obtaining good throughput
- in buffered file read, random I/O throughput is often obtained
only if they are submitted concurrently from lots of tasks; but for
sequential I/O, most of times they can be hit from page cache, so
concurrent submissions often introduce unnecessary context switch
and can't improve throughput much. There was such discussion[1]
to use non-blocking I/O to improve the problem for application.
- with direct I/O and AIO, concurrent submissions can be
avoided and random read throughput can't be affected meantime

So this patchset trys to improve loop via AIO, and about 45% memory
usage can be decreased, see detailed data in commit log of patch4,
also IO throughput isn't affected too.

V5:
	- don't introduce IOCB_DONT_DIRTY_PAGE and bypass dirtying
	for ITER_KVEC and ITER_BVEC direct IO(read), as required by
	Christoph

V4:
	- add detailed commit log for 'use kthread_work'
	- allow userspace(sysfs, losetup) to decide if dio/aio is
	used as suggested by Christoph and Dave Chinner
	- only use dio if the backing block device's min io size
	is 512 as pointed by Dave Chinner & Christoph
V3:
	- based on Al's iov_iter work and Christoph's kiocb changes
	- use kthread_work
	- introduce IOCB_DONT_DIRTY_PAGE flag
	- set QUEUE_FLAG_NOMERGES for loop's request queue
V2:
	- remove 'extra' parameter to aio_kernel_alloc()
	- try to avoid memory allcation inside queue req callback
	- introduce 'use_mq' sysfs file for enabling kernel aio or disabling it
V1:
	- link:
		http://marc.info/?t=140803157700004&r=1&w=2
	- improve failure path in aio_kernel_submit()

 drivers/block/loop.c | 229 ++++++++++++++++++++++++++++++++++++++++++++++++++----------------
 drivers/block/loop.h |  13 ++--
 fs/direct-io.c       |   9 ++-
 3 files changed, 186 insertions(+), 65 deletions(-)

Thanks,
Ming

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