[PATCHSET 0/4] fs: Support for LOOKUP_CACHED / RESOLVE_CACHED

[Jens Axboe <axboe@kernel.dk>]

Hi,

Here's v3 of the LOOKUP_CACHED change. It's exposed as both a flag for
openat2(), and it's used internally by io_uring to speed up (and make more
efficient) the openat/openat2 support there. As posted in the v3 thread,
performance numbers for various levels of the filename lookup already
being cached:

Cached		5.10-git	5.10-git+LOOKUP_CACHED	Speedup
---------------------------------------------------------------
33%		1,014,975	900,474			1.1x
89%		 545,466	292,937			1.9x
100%		 435,636	151,475			2.9x

The more cache hot we are, the faster the inline LOOKUP_CACHED
optimization helps. This is unsurprising and expected, as a thread
offload becomes a more dominant part of the total overhead. If we look
at io_uring tracing, doing an IORING_OP_OPENAT on a file that isn't in
the dentry cache will yield:

275.550481: io_uring_create: ring 00000000ddda6278, fd 3 sq size 8, cq size 16, flags 0
275.550491: io_uring_submit_sqe: ring 00000000ddda6278, op 18, data 0x0, non block 1, sq_thread 0
275.550498: io_uring_queue_async_work: ring 00000000ddda6278, request 00000000c0267d17, flags 69760, normal queue, work 000000003d683991
275.550502: io_uring_cqring_wait: ring 00000000ddda6278, min_events 1
275.550556: io_uring_complete: ring 00000000ddda6278, user_data 0x0, result 4

which shows a failed nonblock lookup, then punt to worker, and then we
complete with fd == 4. This takes 65 usec in total. Re-running the same
test case again:

281.253956: io_uring_create: ring 0000000008207252, fd 3 sq size 8, cq size 16, flags 0
281.253967: io_uring_submit_sqe: ring 0000000008207252, op 18, data 0x0, non block 1, sq_thread 0
281.253973: io_uring_complete: ring 0000000008207252, user_data 0x0, result 4

shows the same request completing inline, also returning fd == 4. This
takes 6 usec.

Using openat2, we see that an attempted RESOLVE_CACHED open of an uncached
file will fail with -EAGAIN, and a subsequent attempt will too as it's
still not cached. ls the file and retry, and we successfully open it
with RESOLVE_CACHED:

[test@archlinux ~]$ ./openat2-cached /etc/nanorc
open: -1
openat2: Resource temporarily unavailable
[test@archlinux ~]$ ./openat2-cached /etc/nanorc
open: -1
openat2: Resource temporarily unavailable
[test@archlinux ~]$ ls -al /etc/nanorc
-rw-r--r-- 1 root root 10066 Dec 17 16:15 /etc/nanorc
[test@archlinux ~]$ ./openat2-cached /etc/nanorc
open: 3

Minor polish since v3:

- Rename LOOKUP_NONBLOCK -> LOOKUP_CACHED, and ditto for the RESOLVE_
  flag. This better explains what the feature does, making it more self
  explanatory in terms of both code readability and for the user visible
  part.

- Remove dead LOOKUP_NONBLOCK check after we've dropped LOOKUP_RCU
  already, spotted by Al.

- Add O_TMPFILE to the checks upfront, so we can drop the checking in
  do_tmpfile().

-- 
Jens Axboe