btrfs: prepare subpage operations to support more than BITS_PER_LONG sub-bitmaps

Browse source

[CURRENT LIMIT]
Btrfs currently only supports sub-bitmaps (e.g. dirty bitmap) no larger
than BITS_PER_LONG.

That limit allows us to easily grab an unsigned long without the need to
properly allocate memory for a larger bitmap.

Unfortunately that limit prevents us from supporting huge folios.
For 4K page size and block size, a huge folio (order 9) means 512 blocks
inside a 2M folio.

[ENHANCEMENT]
To allow direct bitmap operations without allocating new memory,
introduce two different ways to access the subpage bitmaps:

- Return an unsigned long value
  This only happens if blocks_per_folio <= BITS_PER_LONG.

  We read out the sub-bitmap into an unsigned long, and return the
  value.
  This is the old existing method.

  This involves get_bitmap_value_##name() helper functions.
  And this time the helper functions are defined as inline functions
  instead of macros to provide better type checks.

- Return a pointer where the sub-bitmap starts
  This only happens if blocks_per_folio >= BITS_PER_LONG.

  This is the new method for sub-bitmaps larger than BITS_PER_LONG.
  Since the sizes of sub-bitmaps are all aligned to BITS_PER_LONG, we
  can directly access the start word of the sub-bitmap.

  This involves get_bitmap_pointer_##name() helper functions.

Then change the existing sub-bitmaps users to use the new helpers:

- Bitmap dumping
  Switch between get_bitmap_value_##name() and
  get_bitmap_pointer_##name() depending on the sub-bitmap size.

- btrfs_get_subpage_dirty_bitmap()
  Rename it to btrfs_get_subpage_dirty_bitmap_value() to follow the new
  value/pointer naming.
  Since we do not support huge folios yet, there is no pointer version
  for the dirty bitmap.

  Furthermore, add the support for block size == page size cases for
  btrfs_get_subpage_dirty_bitmap_value(), so that the caller no longer
  needs to check if the folio needs subpage handling.

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

...

This commit is contained in:

Qu Wenruo 2026-05-13 14:06:19 +09:30 • committed by Filipe Manana

parent a93a87780d

commit eb6915bb86

3 changed files with 87 additions and 43 deletions

Show all changes Ignore whitespace when comparing lines Ignore changes in amount of whitespace Ignore changes in whitespace at EOL

Download patch file Download diff file Expand all files Collapse all files

7

fs/btrfs/extent_io.c

View file

118

fs/btrfs/subpage.c

View file

5

fs/btrfs/subpage.h

View file