btrfs: preallocate temporary extent buffer for inode logging when needed

btrfs: preallocate temporary extent buffer for inode logging when needed

When logging an inode and we require to copy items from subvolume leaves
to the log tree, we clone each subvolume leaf and than use that clone to
copy items to the log tree. This is required to avoid possible deadlocks
as stated in commit 796787c978ef ("btrfs: do not modify log tree while
holding a leaf from fs tree locked").

The cloning requires allocating an extent buffer (struct extent_buffer)
and then allocating pages (folios) to attach to the extent buffer. This
may be slow in case we are under memory pressure, and since we are doing
the cloning while holding a read lock on a subvolume leaf, it means we
can be blocking other operations on that leaf for significant periods of
time, which can increase latency on operations like creating other files,
renaming files, etc. Similarly because we're under a log transaction, we
may also cause extra delay on other tasks doing an fsync, because syncing
the log requires waiting for tasks that joined a log transaction to exit
the transaction.

So to improve this, for any inode logging operation that needs to copy
items from a subvolume leaf ("full sync" or "copy everything" bit set
in the inode), preallocate a dummy extent buffer before locking any
extent buffer from the subvolume tree, and even before joining a log
transaction, add it to the log context and then use it when we need to
copy items from a subvolume leaf to the log tree. This avoids making
other operations get extra latency when waiting to lock a subvolume
leaf that is used during inode logging and we are under heavy memory
pressure.

The following test script with bonnie++ was used to test this:

  $ cat test.sh
  #!/bin/bash

  DEV=/dev/sdh
  MNT=/mnt/sdh
  MOUNT_OPTIONS="-o ssd"

  MEMTOTAL_BYTES=`free -b | grep Mem: | awk '{ print $2 }'`
  NR_DIRECTORIES=20
  NR_FILES=20480
  DATASET_SIZE=$((MEMTOTAL_BYTES * 2 / 1048576))
  DIRECTORY_SIZE=$((MEMTOTAL_BYTES * 2 / NR_FILES))
  NR_FILES=$((NR_FILES / 1024))

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

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

  bonnie++ -u root -d $MNT \
      -n $NR_FILES:$DIRECTORY_SIZE:$DIRECTORY_SIZE:$NR_DIRECTORIES \
      -r 0 -s $DATASET_SIZE -b

  umount $MNT

The results of this test on a 8G VM running a non-debug kernel (Debian's
default kernel config), were the following.

Before this change:

  Version 2.00a       ------Sequential Output------ --Sequential Input- --Random-
                      -Per Chr- --Block-- -Rewrite- -Per Chr- --Block-- --Seeks--
  Name:Size etc        /sec %CP  /sec %CP  /sec %CP  /sec %CP  /sec %CP  /sec %CP
  debian0       7501M  376k  99  1.4g  96  117m  14 1510k  99  2.5g  95 +++++ +++
  Latency             35068us   24976us    2944ms   30725us   71770us   26152us
  Version 2.00a       ------Sequential Create------ --------Random Create--------
  debian0             -Create-- --Read--- -Delete-- -Create-- --Read--- -Delete--
  files:max:min        /sec %CP  /sec %CP  /sec %CP  /sec %CP  /sec %CP  /sec %CP
  20:384100:384100/20 20480  32 20480  58 20480  48 20480  39 20480  56 20480  61
  Latency               411ms   11914us     119ms     617ms   10296us     110ms

After this change:

  Version 2.00a       ------Sequential Output------ --Sequential Input- --Random-
                      -Per Chr- --Block-- -Rewrite- -Per Chr- --Block-- --Seeks--
  Name:Size etc        /sec %CP  /sec %CP  /sec %CP  /sec %CP  /sec %CP  /sec %CP
  debian0       7501M  375k  99  1.4g  97  117m  14 1546k  99  2.3g  98 +++++ +++
  Latency             35975us  20945us    2144ms   10297us    2217us    6004us
  Version 2.00a       ------Sequential Create------ --------Random Create--------
  debian0             -Create-- --Read--- -Delete-- -Create-- --Read--- -Delete--
  files:max:min        /sec %CP  /sec %CP  /sec %CP  /sec %CP  /sec %CP  /sec %CP
  20:384100:384100/20 20480  35 20480  58 20480  48 20480  40 20480  57 20480  59
  Latency               320ms   11237us   77779us     518ms    6470us   86389us

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>