During a send operation, when doing backref walking to determine which
inodes/offsets/roots we can clone from, the most repetitive and expensive
step is to map each leaf that has file extent items pointing to the target
data extent to the IDs of the roots from which the leaves are accessible,
which happens at iterate_extent_inodes(). That step requires finding every
parent node of a leaf, then the parent of each parent, and so on until we
reach a root node. So it's a naturally expensive operation, and repetitive
because each leaf can have hundreds of file extent items (for a nodesize
of 16K, that can be slightly over 200 file extent items). There's also
temporal locality, as we process all file extent items from a leave before
moving the next leaf.

This change caches the mapping of leaves to root IDs, to avoid repeating
those computations over and over again. The cache is limited to a maximum
of 128 entries, with each entry being a struct with a size of 128 bytes,
so the maximum cache size is 16K plus any nodes internally allocated by
the maple tree that is used to index pointers to those structs. The cache
is invalidated whenever we detect relocation happened since we started
filling the cache, because if relocation happened then extent buffers for
leaves and nodes of the trees used by a send operation may have been
reallocated.

This cache also allows for another important optimization that is
introduced in the next patch in the series.

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

  01/17 btrfs: fix inode list leak during backref walking at resolve_indirect_refs()
  02/17 btrfs: fix inode list leak during backref walking at find_parent_nodes()
  03/17 btrfs: fix ulist leaks in error paths of qgroup self tests
  04/17 btrfs: remove pointless and double ulist frees in error paths of qgroup tests
  05/17 btrfs: send: avoid unnecessary path allocations when finding extent clone
  06/17 btrfs: send: update comment at find_extent_clone()
  07/17 btrfs: send: drop unnecessary backref context field initializations
  08/17 btrfs: send: avoid unnecessary backref lookups when finding clone source
  09/17 btrfs: send: optimize clone detection to increase extent sharing
  10/17 btrfs: use a single argument for extent offset in backref walking functions
  11/17 btrfs: use a structure to pass arguments to backref walking functions
  12/17 btrfs: reuse roots ulist on each leaf iteration for iterate_extent_inodes()
  13/17 btrfs: constify ulist parameter of ulist_next()
  14/17 btrfs: send: cache leaf to roots mapping during backref walking
  15/17 btrfs: send: skip unnecessary backref iterations
  16/17 btrfs: send: avoid double extent tree search when finding clone source
  17/17 btrfs: send: skip resolution of our own backref when finding clone source

Performance test results are in the changelog of patch 17/17.
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>