At btrfs_record_unlink_dir() we directly check the logged_trans field of
the given inodes to check if they were previously logged in the current
transaction, and if any of them were, then we can avoid setting the field
last_unlink_trans of the directory to the id of the current transaction if
we are in a rename path. Avoiding that can later prevent falling back to
a transaction commit if anyone attempts to log the directory.

However the logged_trans field, store in struct btrfs_inode, is transient,
not persisted in the inode item on its subvolume b+tree, so that means
that if an inode is evicted and then loaded again, its original value is
lost and it's reset to 0. So directly checking the logged_trans field can
lead to some false negative, and that only results in a performance impact
as mentioned before.

Instead of directly checking the logged_trans field of the inodes, use the
inode_logged() helper, which will check in the log tree if an inode was
logged before in case its logged_trans field has a value of 0. This way
we can avoid setting the directory inode's last_unlink_trans and cause
future logging attempts of it to fallback to transaction commits. The
following test script shows one example where this happens without this
patch:

  $ cat test.sh
  #!/bin/bash

  DEV=/dev/nullb0
  MNT=/mnt/nullb0

  num_init_files=10000
  num_new_files=10000

  mkfs.btrfs -f $DEV
  mount -o ssd $DEV $MNT

  mkdir $MNT/testdir
  for ((i = 1; i <= $num_init_files; i++)); do
      echo -n > $MNT/testdir/file_$i
   done

  echo -n > $MNT/testdir/foo

  sync

  # Add some files so that there's more work in the transaction other
  # than just renaming file foo.
  for ((i = 1; i <= $num_new_files; i++)); do
      echo -n > $MNT/testdir/new_file_$i
  done

  # Change the file, fsync it.
  setfattr -n user.x1 -v 123 $MNT/testdir/foo
  xfs_io -c "fsync" $MNT/testdir/foo

  # Now triggger eviction of file foo but no eviction for our test
  # directory, since it is being used by the process below. This will
  # set logged_trans of the file's inode to 0 once it is loaded again.
  (
      cd $MNT/testdir
      while true; do
          :
      done
  ) &
  pid=$!

  echo 2 > /proc/sys/vm/drop_caches

  kill $pid
  wait $pid

  # Move foo out of our testdir. This will set last_unlink_trans
  # of the directory inode to the current transaction, because
  # logged_trans of both the directory and the file are set to 0.
  mv $MNT/testdir/foo $MNT/foo

  # Change file foo again and fsync it.
  # This fsync will result in a transaction commit because the rename
  # above has set last_unlink_trans of the parent directory to the id
  # of the current transaction and because our inode for file foo has
  # last_unlink_trans set to the current transaction, since it was
  # evicted and reloaded and it was previously modified in the current
  # transaction (the xattr addition).
  xfs_io -c "pwrite 0 64K" $MNT/foo
  start=$(date +%s%N)
  xfs_io -c "fsync" $MNT/foo
  end=$(date +%s%N)
  dur=$(( (end - start) / 1000000 ))

  echo "file fsync took: $dur milliseconds"

  umount $MNT

Before this patch:   fsync took 19 milliseconds
After this patch:    fsync took  5 milliseconds
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>