[PATCH v2 0/3] btrfs: make subpage reader/writer counter to be sector aware

[Qu Wenruo <wqu@suse.com>]

[CHANGELOG]
v2:
- Add btrfs_subpage_dump_bitmap() support for locked bitmap

- Add spinlock to protect the bitmap and locked bitmap operation
  In theory, this opeartion should always be single threaded, since the
  page is locked.
  But to keep the behavior consistent, use spin lock to protect bitmap
  and atomic reader/write updates.

This can be fetched from github, and the branch would be utilized for
all newer subpage delalloc update to support full sector sized
compression and zoned:
https://github.com/adam900710/linux/tree/subpage_delalloc

Currently we just trace subpage reader/writer counter using an atomic.

It's fine for the current subpage usage, but for the future, we want to
be aware of which subpage sector is locked inside a page, for proper
compression (we only support full page compression for now) and zoned support.

So here we introduce a new bitmap, called locked bitmap, to trace which
sector is locked for read/write.

And since reader/writer are both exclusive (to each other and to the same
type of lock), we can safely use the same bitmap for both reader and
writer.

In theory we can use the bitmap (the weight of the locked bitmap) to
indicate how many bytes are under reader/write lock, but it's not
possible yet:

- No weight support for bitmap range
  The bitmap API only provides bitmap_weight(), which always starts at
  bit 0.

- Need to distinguish read/write lock

Thus we still keep the reader/writer atomic counter.

Qu Wenruo (3):
  btrfs: unexport btrfs_subpage_start_writer() and
    btrfs_subpage_end_and_test_writer()
  btrfs: subpage: make reader lock to utilize bitmap
  btrfs: subpage: make writer lock to utilize bitmap

 fs/btrfs/subpage.c | 77 ++++++++++++++++++++++++++++++++++++----------
 fs/btrfs/subpage.h | 16 +++++++---
 2 files changed, 72 insertions(+), 21 deletions(-)

-- 
2.43.1