BM_BLOCK_MASK
((pfn - zone->start_pfn) & ~BM_BLOCK_MASK) == bm->cur.node_pfn)
bm->cur.node_pfn = (pfn - zone->start_pfn) & ~BM_BLOCK_MASK;
*bit_nr = (pfn - zone->start_pfn) & BM_BLOCK_MASK;