btrfs: compression: refactor and enhancement preparing for subpage compression support

There are quite some problems in compression code:

- Weird compressed_bio::pending_bios dance
  If we just don't want compressed_bio being freed halfway, we have more
  sane methods, just like btrfs_subpage::readers.

  So here we fix it by introducing compressed_bio::io_sectors to do the
  job.

- BUG_ON()s inside btrfs_submit_compressed_*()
  Even they are just ENOMEM, we should handle them.
  With io_sectors introduced, we have a way to finish compressed_bio
  all by ourselves, as long as we haven't submitted last bio.

  If we have last bio submitted, then endio will handle it well.

- Duplicated code for compressed bio allocation and submission
  Just small refactor can handle it

- Stripe boundary is checked every time one page is added
  This is overkilled.
  Just learn from extent_io.c refactor which use bio_ctrl to do the
  boundary check only once for each bio.

  Although in compression context, we don't need extra checks in
  extent_io.c, thus we don't need bio_ctrl structure, but
  can afford to do it locally.

- Dead code removal
  One dead comment and a new zombie function,
  btrfs_bio_fits_in_stripe(), can be removed now.

Changelog:
v2:
- Rebased to latest misc-next

- Fix a bug in btrfs_submit_compressed_write() where zoned write is not
  taken into consideration

- Reuse the existing chunk mapping of btrfs_get_chunk_map()

v3:
- Fix a bug that zoned device can't even pass btrfs/001
  This is because at endio time, bi_size for zoned device is always 0.
  We have to use bio_for_each_segment_all() to calculate the real bio
  size instead.
  In theory, it should also happen more frequently for non-zoned device,
  but no catch for all test cases (with "-o compress") except btrfs/011.

- Fix btrfs/011 hang when tested with "-o compress"
  This is caused by checking both atomic value without protection.
  Checking two atomic values is no longer atomic.

  In fact, with compressed_bio::io_sectors introduced, pending_bios is
  only used to wait for any pending bio to finish in error path.

  Thus dec_and_test_compressed_bio() only need to check if io_sectors is
  zero

- Fix a error that in error handling path, we may hang due to missing
  wake_up() in dec_and_test_compressed_bio()

v4:
- Use formal words for BUG_ON() removal patch titles

- Remove compressed_bio::pending_bios
  As compressed_bio::pending_sectors can replace it completely

- Remove unnecessary comments and BUG_ON()s

- Use wait_var_event() APIs to reduce the memory overhead

- Comments update to follow the same schema for moved comments

Qu Wenruo (9):
  btrfs: remove a dead comment for btrfs_decompress_bio()
  btrfs: introduce compressed_bio::pending_sectors to trace compressed
    bio more elegantly
  btrfs: handle errors properly inside btrfs_submit_compressed_read()
  btrfs: handle errors properly inside btrfs_submit_compressed_write()
  btrfs: introduce submit_compressed_bio() for compression
  btrfs: introduce alloc_compressed_bio() for compression
  btrfs: make btrfs_submit_compressed_read() to determine stripe
    boundary at bio allocation time
  btrfs: make btrfs_submit_compressed_write() to determine stripe
    boundary at bio allocation time
  btrfs: remove unused function btrfs_bio_fits_in_stripe()

 fs/btrfs/compression.c | 587 +++++++++++++++++++++++------------------
 fs/btrfs/compression.h |   4 +-
 fs/btrfs/ctree.h       |   2 -
 fs/btrfs/inode.c       |  42 ---
 4 files changed, 336 insertions(+), 299 deletions(-)

-- 
2.32.0