ARTIFICIAL_VNID_BITS
#define IS_DIR_INDEX_VNID(vnid) (((vnid) & ARTIFICIAL_VNID_BITS) == DIR_INDEX_VNID_BITS)
#define IS_ARTIFICIAL_VNID(vnid) (((vnid) & ARTIFICIAL_VNID_BITS) == ARTIFICIAL_VNID_BITS)
vol->vcache.cur_vnid = ARTIFICIAL_VNID_BITS;