When doing a NOWAIT direct IO write, if we can NOCOW then it means we can
proceed with the non-blocking, NOWAIT path. However reserving the metadata
space and qgroup meta space can often result in blocking - flushing
delalloc, wait for ordered extents to complete, trigger transaction
commits, etc, going against the semantics of a NOWAIT write.

So make the NOWAIT write path to try to reserve all the metadata it needs
without resulting in a blocking behaviour - if we get -ENOSPC or -EDQUOT
then return -EAGAIN to make the caller fallback to a blocking direct IO
write.

This is part of a patchset comprised of the following patches:

  btrfs: avoid blocking on page locks with nowait dio on compressed range
  btrfs: avoid blocking nowait dio when locking file range
  btrfs: avoid double nocow check when doing nowait dio writes
  btrfs: stop allocating a path when checking if cross reference exists
  btrfs: free path at can_nocow_extent() before checking for checksum items
  btrfs: release path earlier at can_nocow_extent()
  btrfs: avoid blocking when allocating context for nowait dio read/write
  btrfs: avoid blocking on space revervation when doing nowait dio writes

The following test was run before and after applying this patchset:

  $ cat io-uring-nodatacow-test.sh
  #!/bin/bash

  DEV=/dev/sdc
  MNT=/mnt/sdc

  MOUNT_OPTIONS="-o ssd -o nodatacow"
  MKFS_OPTIONS="-R free-space-tree -O no-holes"

  NUM_JOBS=4
  FILE_SIZE=8G
  RUN_TIME=300

  cat <<EOF > /tmp/fio-job.ini
  [io_uring_rw]
  rw=randrw
  fsync=0
  fallocate=posix
  group_reporting=1
  direct=1
  ioengine=io_uring
  iodepth=64
  bssplit=4k/20:8k/20:16k/20:32k/10:64k/10:128k/5:256k/5:512k/5:1m/5
  filesize=$FILE_SIZE
  runtime=$RUN_TIME
  time_based
  filename=foobar
  directory=$MNT
  numjobs=$NUM_JOBS
  thread
  EOF

  echo performance | \
     tee /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor

  umount $MNT &> /dev/null
  mkfs.btrfs -f $MKFS_OPTIONS $DEV &> /dev/null
  mount $MOUNT_OPTIONS $DEV $MNT

  fio /tmp/fio-job.ini

  umount $MNT

The test was run a 12 cores box with 64G of ram, using a non-debug kernel
config (Debian's default config) and a spinning disk.

Result before the patchset:

 READ: bw=407MiB/s (427MB/s), 407MiB/s-407MiB/s (427MB/s-427MB/s), io=119GiB (128GB), run=300175-300175msec
WRITE: bw=407MiB/s (427MB/s), 407MiB/s-407MiB/s (427MB/s-427MB/s), io=119GiB (128GB), run=300175-300175msec

Result after the patchset:

 READ: bw=436MiB/s (457MB/s), 436MiB/s-436MiB/s (457MB/s-457MB/s), io=128GiB (137GB), run=300044-300044msec
WRITE: bw=435MiB/s (456MB/s), 435MiB/s-435MiB/s (456MB/s-456MB/s), io=128GiB (137GB), run=300044-300044msec

That's about +7.2% throughput for reads and +6.9% for writes.
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>