[PATCH 0/3] btrfs: reduce memory usage for btrfs_raid_bio structure

[Qu Wenruo <wqu@suse.com>]

This series replace the following members of btrfs_raid_bio:

	struct sector_ptr bio_sectors[nr_sectors]
	struct sector_ptr stripe_sectors[nr_sectors]

To the following ones:

	phys_addr_t bio_sectors[nr_sectors]
	phys_addr_t stripe_sectors[nr_sectors]
	unsigned long uptodate_bitmap[nr_sectors]

For x86_64 (4K page size) with the fixed 64K stripe size and 3 disks, the
memory usage of those members (not the full structure) will be reduced from:

	8 * 2 + 48 * 16 * 2 = 1552

To
	8 * 3 + 48 * 8 * 2 + 8 * 2 = 808

Almost halved the memory usage.

The memory saving comes from:

- Compat sector_ptr::uptodate bit into a bitmap
  This not only saves space, but also allow us to call bitmap_*()
  helpers when we need to set multiple bits in one go (mostly for
  subpage cases)

- Remove sector_ptr::uptodate flag
  We can use a special paddr (not NULL though) to indicate if the paddr
  is valid or not.

- Get rid of sector_ptr
  That structure has extra bits that take a full byte for each flag.
  This is the biggest space saving.

Qu Wenruo (3):
  btrfs: raid56: remove sector_ptr::has_paddr member
  btrfs: raid56: move sector_ptr::uptodate into a dedicated bitmap
  btrfs: raid56: remove sector_ptr structure

 fs/btrfs/raid56.c | 347 ++++++++++++++++++++++------------------------
 fs/btrfs/raid56.h |  17 +--
 2 files changed, 168 insertions(+), 196 deletions(-)

-- 
2.50.1