btrfs: remove locked subpage bitmap

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Currently there are two members inside btrfs_folio_state that are related
to locked bitmap:

- locked sub-bitmap inside btrfs_folio_state::bitmaps[]
  The enum btrfs_bitmap_nr_locked determines the sub-bitmap.

- btrfs_folio_state::nr_locked
  Which records how many blocks are locked inside the folio.

The locked sub-bitmap is a btrfs specific per-block tracking mechanism,
which is mostly for async-submission, utilized by compressed writes.

The sub-bitmap itself is a super set of nr_locked, as it can provide a
more reliable tracking.

But the sub-bitmap itself can be pretty large for the incoming huge
folio, 2M sized folio for 4K page size, meaning 512 bits for one
sub-bitmap.

Furthermore, in the long run compression will be reworked to get rid of
async-submission completely, there is not much need for a full
sub-bitmap to track the locked status.

This patch removes the locked sub-bitmap and only relies on @nr_locked
atomic to do the tracking.
This can also save 64 bytes from btrfs_folio_state::bitmaps[] for a huge
folio.

This will reduce some safety checks, as previously if a block is not
locked, btrfs_folio_end_lock()/btrfs_folio_end_lock_bitmap() will find
out that, and skip reducing @nr_locked for that block, and avoid
under-flow.

But this safety net itself shouldn't be necessary in the first place.
If we're unlocking a block that is not locked, it's a bug in the logic,
and we should catch it, not silently ignoring it.
Thus I believe the removal of the extra safety net should not be a
problem.

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

...

This commit is contained in:

Qu Wenruo 2026-05-09 18:36:31 +09:30 • committed by David Sterba

parent 0af37c217e

commit 0e7fff6eca

2 changed files with 10 additions and 45 deletions

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45

fs/btrfs/subpage.c

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10

fs/btrfs/subpage.h

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