btrfs: refactor how we handle reserved space inside copy_one_range()

There are several things not ideal in copy_one_range():

- Unnecessary temporary variables
  * block_offset
  * reserve_bytes
  * dirty_blocks
  * num_blocks
  * release_bytes
  These are utilized to handle short-copy cases.

- Inconsistent handling of btrfs_delalloc_release_extents()
  There is a hidden behavior that, after reserving metadata for X bytes
  of data write, we have to call btrfs_delalloc_release_extents() with X
  once and only once.

  Calling btrfs_delalloc_release_extents(X - 4K) and
  btrfs_delalloc_release_extents(4K) will cause outstanding extents
  accounting to go wrong.

  This is because the outstanding extents mechanism is not designed to
  handle shrinking of reserved space.

Improve above situations by:

- Use a single @reserved_start and @reserved_len pair
  Now we reserve space for the initial range, and if a short copy
  happened and we need to shrink the reserved space, we can easily
  calculate the new length, and update @reserved_len.

- Introduce helpers to shrink reserved data and metadata space
  This is done by two new helpers, shrink_reserved_space() and
  btrfs_delalloc_shrink_extents().

  The later will do a better calculation if we need to modify the
  outstanding extents, and the first one will be utilized inside
  copy_one_range().

- Manually unlock, release reserved space and return if no byte is
  copied

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

1 files changed, 1 insertions, 0 deletions

diff --git a/fs/btrfs/delalloc-space.h b/fs/btrfs/delalloc-space.h
index 3f32953c0a80..069005959479 100644
--- a/fs/btrfs/delalloc-space.h
+++ b/fs/btrfs/delalloc-space.h
@@ -27,5 +27,6 @@ int btrfs_delalloc_reserve_space(struct btrfs_inode *inode,
 int btrfs_delalloc_reserve_metadata(struct btrfs_inode *inode, u64 num_bytes,
 				    u64 disk_num_bytes, bool noflush);
 void btrfs_delalloc_release_extents(struct btrfs_inode *inode, u64 num_bytes);
+void btrfs_delalloc_shrink_extents(struct btrfs_inode *inode, u64 reserved_len, u64 new_len);
 
 #endif /* BTRFS_DELALLOC_SPACE_H */