[PATCHSET v2 for-next 0/3] Add FMODE_NOWAIT support to pipes

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One thing that's always been a bit slower than I'd like with io_uring is
dealing with pipes. They don't support IOCB_NOWAIT, and hence we need to
punt them to io-wq for handling.

This series adds support for FMODE_NOWAIT to pipes.

Patch 1 extends pipe_buf_operations->confirm() to accept a nonblock
parameter, and wires up the caller, pipe_buf_confirm(), to have that
argument too.

Patch 2 makes pipes deal with IOCB_NOWAIT for locking the pipe, calling
pipe_buf_confirm(), and for allocating new pages on writes.

Patch 3 flicks the switch and enables FMODE_NOWAIT for pipes.

Curious on how big of a difference this makes, I wrote a small benchmark
that simply opens 128 pipes and then does 256 rounds of reading and
writing to them. This was run 10 times, discarding the first run as it's
always a bit slower. Before the patch:

Avg:	262.52 msec
Stdev:	  2.12 msec
Min:	261.07 msec
Max	267.91 msec

and after the patch:

Avg:	24.14 msec
Stdev:	 9.61 msec
Min:	17.84 msec
Max:	43.75 msec

or about a 10x improvement in performance (and efficiency) for pipes
being empty on read attempt. If we run the same test but with pipes
already having data, the improvement is even better (as expected):


Avg:	249.24 msec
Stdev:	  0.20 msec
Min:	248.96 msec
Max:	249.53 msec


Avg:	 10.86 msec
Stdev:	  0.91 msec
Min:	 10.02 msec
Max:	 12.67 msec

or about a 23x improvement.

I ran the patches through the ltp pipe and splice tests, no regressions
observed. Looking at io_uring traces, we can see that we no longer have
any io_uring_queue_async_work() traces after the patch, where previously
everything was done via io-wq.

Changes since v1:
- Add acks/reviewed-bys
- Fix missing __GFP_HARDWALL (willy)
- Get rid of nasty double ternary (willy,christian)

Jens Axboe

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