PGF_ATTR_REF
#define PGF_PRESERVE (PGF_ATTR_MOD | PGF_ATTR_REF)
PGF_ATTR_MOD | PGF_ATTR_REF);
atomic_setbits_int(&pg->pg_flags, PGF_ATTR_REF);
referenced = (pg->pg_flags & PGF_ATTR_REF) != 0;
atomic_clearbits_int(&pg->pg_flags, PGF_ATTR_REF);
return (pg->pg_flags & PGF_ATTR_REF) != 0;
atomic_setbits_int(&pg->pg_flags, PGF_ATTR_MOD | PGF_ATTR_REF);