BM_BITS_PER_BLOCK
rtree = nodes = DIV_ROUND_UP(zone->spanned_pages, BM_BITS_PER_BLOCK);
nr_blocks = DIV_ROUND_UP(pages, BM_BITS_PER_BLOCK);
bm->cur.node_pfn += BM_BITS_PER_BLOCK;
bits = min(pages - bm->cur.node_pfn, BM_BITS_PER_BLOCK);