[PATCH v3 0/3] btrfs: allow btrfs_truncate_block() to fallback to nocow for data space reservation

[Qu Wenruo <wqu@suse.com>]

Before this patch, btrfs_truncate_block() never checks the NODATACOW
bit, thus when we run out of data space, we can return ENOSPC for
truncate for NODATACOW inode.

This patchset will address this problem by doing the same behavior as
buffered write to address it.

Changelog:
v2:
- Rebased to misc-next
  Only one minor conflict in ctree.h

v3:
- Added two new patches
- Refactor check_can_nocow()
  Since the introduction of nowait, check_can_nocow() are in fact split
  into two usage patterns: check_can_nocow(nowait = false) with
  btrfs_drew_write_unlock(), and single check_can_nocow(nowait = true).
  Refactor them into two functions: start_nocow_check() paired with
  end_nocow_check(), and single try_nocow_check(). With comment added.

- Rebased to latest misc-next

- Added btrfs_assert_drew_write_locked() for btrfs_end_nocow_check()
  This is a little concerning one, as it's in the hot path of buffered
  write.
  It has percpu_counter_sum() called in that hot path, causing
  obvious performance drop for CONFIG_BTRFS_DEBUG build.
  Not sure if the assert is worthy since there aren't any other users.

Qu Wenruo (3):
  btrfs: add comments for check_can_nocow() and can_nocow_extent()
  btrfs: refactor check_can_nocow() into two variants
  btrfs: allow btrfs_truncate_block() to fallback to nocow for data
    space reservation

 fs/btrfs/ctree.h   |  3 +++
 fs/btrfs/file.c    | 52 +++++++++++++++++++++++++++++++++------
 fs/btrfs/inode.c   | 61 ++++++++++++++++++++++++++++++++++++++++------
 fs/btrfs/locking.h | 13 ++++++++++
 4 files changed, 113 insertions(+), 16 deletions(-)

-- 
2.27.0