PG_READAHEAD
if ((pg->flags & PG_READAHEAD) != 0) {
pg->flags &= ~PG_READAHEAD;
if ((pg->flags & PG_READAHEAD) != 0) {
pg->flags &= ~PG_READAHEAD;
pg->flags |= PG_READAHEAD;
if ((p->flags & PG_READAHEAD) != 0) {
p->flags &= ~PG_READAHEAD;