[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>