BM_BYTE
#define BM_ISSET(p, vnid) (BM_BYTE(p, vnid) & (1 << (p->nextvnid % sizeof(BMT))))
#define BM_SET(p, vnid) (BM_BYTE(p, vnid) |= (1 << (p->nextvnid % sizeof(BMT))))
#define BM_UNSET(p, vnid) (BM_BYTE(p, vnid) &= ~(1 << (p->nextvnid % sizeof(BMT))))