btrfs: async discard support

Hello,

Dave reported a lockdep issue [1]. I'm a bit surprised as I can't repro
it, but it obviously is right. I believe I fixed the issue by moving the
fully trimmed check outside of the block_group lock.  I mistakingly
thought the btrfs_block_group lock subsumed btrfs_free_space_ctl
tree_lock. This clearly isn't the case.

Changes in v6:
 - Move the fully trimmed check outside of the block_group lock.

v5 is available here: [2].

This series is on top of btrfs-devel#misc-next 7ee98bb808e2 + [3] and
[4].

[1] https://lore.kernel.org/linux-btrfs/20191210140438.GU2734...
[2] https://lore.kernel.org/linux-btrfs/cover.1575919745.git....
[3] https://lore.kernel.org/linux-btrfs/d934383ea528d920a95b6...
[4] https://lore.kernel.org/linux-btrfs/20191209193846.18162-...

Dennis Zhou (22):
  bitmap: genericize percpu bitmap region iterators
  btrfs: rename DISCARD opt to DISCARD_SYNC
  btrfs: keep track of which extents have been discarded
  btrfs: keep track of cleanliness of the bitmap
  btrfs: add the beginning of async discard, discard workqueue
  btrfs: handle empty block_group removal
  btrfs: discard one region at a time in async discard
  btrfs: add removal calls for sysfs debug/
  btrfs: make UUID/debug have its own kobject
  btrfs: add discard sysfs directory
  btrfs: track discardable extents for async discard
  btrfs: keep track of discardable_bytes
  btrfs: calculate discard delay based on number of extents
  btrfs: add bps discard rate limit
  btrfs: limit max discard size for async discard
  btrfs: make max async discard size tunable
  btrfs: have multiple discard lists
  btrfs: only keep track of data extents for async discard
  btrfs: keep track of discard reuse stats
  btrfs: add async discard header
  btrfs: increase the metadata allowance for the free_space_cache
  btrfs: make smaller extents more likely to go into bitmaps

 fs/btrfs/Makefile           |   2 +-
 fs/btrfs/block-group.c      |  87 ++++-
 fs/btrfs/block-group.h      |  30 ++
 fs/btrfs/ctree.h            |  52 ++-
 fs/btrfs/discard.c          | 684 ++++++++++++++++++++++++++++++++++++
 fs/btrfs/discard.h          |  42 +++
 fs/btrfs/disk-io.c          |  15 +-
 fs/btrfs/extent-tree.c      |   8 +-
 fs/btrfs/free-space-cache.c | 611 +++++++++++++++++++++++++++-----
 fs/btrfs/free-space-cache.h |  41 ++-
 fs/btrfs/inode-map.c        |  13 +-
 fs/btrfs/inode.c            |   2 +-
 fs/btrfs/scrub.c            |   7 +-
 fs/btrfs/super.c            |  39 +-
 fs/btrfs/sysfs.c            | 205 ++++++++++-
 fs/btrfs/volumes.c          |   7 +
 include/linux/bitmap.h      |  35 ++
 mm/percpu.c                 |  61 +---
 18 files changed, 1789 insertions(+), 152 deletions(-)
 create mode 100644 fs/btrfs/discard.c
 create mode 100644 fs/btrfs/discard.h

Thanks,
Dennis