BTRFS_FREE_SPACE_BITMAP_SIZE   32 fs/btrfs/free-space-tree.c 	bitmap_size = sizeof(struct btrfs_item) + BTRFS_FREE_SPACE_BITMAP_SIZE;
BTRFS_FREE_SPACE_BITMAP_SIZE   17 fs/btrfs/free-space-tree.h #define BTRFS_FREE_SPACE_BITMAP_BITS (BTRFS_FREE_SPACE_BITMAP_SIZE * BITS_PER_BYTE)