btrfs: send: reduce searches on parent root when checking if dir can be removed

btrfs: send: reduce searches on parent root when checking if dir can be removed

During an incremental send, every time we remove a reference (dentry) for
an inode and the parent directory does not exists anymore in the send
root, we go check if we can remove the directory by making a call to
can_rmdir(). This helper can only return true (value 1) if all dentries
were already removed, and for that it always does a search on the parent
root for dir index keys - if it finds any dentry referring to an inode
with a number higher then the inode currently being processed, then the
directory can not be removed and it must return false (value 0).

However that means if a directory that was deleted had 1000 dentries, and
each one pointed to an inode with a number higher then the number of the
directory's inode, we end up doing 1000 searches on the parent root.
Typically files are created in a directory after the directory was created
and therefore they get an higher inode number than the directory. It's
also common to have the each dentry pointing to an inode with a higher
number then the inodes the previous dentries point to, for example when
creating a series of files inside a directory, a very common pattern.

So improve on that by having the first call to can_rmdir() for a directory
to check the number of the inode that the last dentry points to and cache
that inode number in the orphan dir structure. Then every subsequent call
to can_rmdir() can avoid doing a search on the parent root if the number
of the inode currently being processed is smaller than cached inode number
at the directory's orphan dir structure.

This patch is part of a larger patchset and the changelog of the last
patch in the series contains a sample performance test and results.
The patches that comprise the patchset are the following:

  btrfs: send: directly return from did_overwrite_ref() and simplify it
  btrfs: send: avoid unnecessary generation search at did_overwrite_ref()
  btrfs: send: directly return from will_overwrite_ref() and simplify it
  btrfs: send: avoid extra b+tree searches when checking reference overrides
  btrfs: send: remove send_progress argument from can_rmdir()
  btrfs: send: avoid duplicated orphan dir allocation and initialization
  btrfs: send: avoid unnecessary orphan dir rbtree search at can_rmdir()
  btrfs: send: reduce searches on parent root when checking if dir can be removed
  btrfs: send: iterate waiting dir move rbtree only once when processing refs
  btrfs: send: initialize all the red black trees earlier
  btrfs: send: genericize the backref cache to allow it to be reused
  btrfs: adapt lru cache to allow for 64 bits keys on 32 bits systems
  btrfs: send: cache information about created directories
  btrfs: allow a generation number to be associated with lru cache entries
  btrfs: add an api to delete a specific entry from the lru cache
  btrfs: send: use the lru cache to implement the name cache
  btrfs: send: update size of roots array for backref cache entries
  btrfs: send: cache utimes operations for directories if possible

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