checkpage
if ((rvfront = checkpage(fhand, POH_FRONT)) == CKP_FREED) {
if ((rvback = checkpage(bhand, POH_BACK)) == CKP_FREED) {
VM_STAT_ADD(pageoutvmstats.checkpage[0]);
VM_STAT_ADD(pageoutvmstats.checkpage[1]);
VM_STAT_ADD(pageoutvmstats.checkpage[2]);
ulong_t checkpage[3];
static checkpage_result_t checkpage(page_t *, pageout_hand_t);