title: Page PG_referenced vs PG_active bit?
body: <p>I was reading about pages and memory allocaitons and found:</p>
  <p><code>PG_referenced</code> and <code>PG_active</code> but it's not clear to me what's the difference at all?
  I know that PG_active is set when acessing the page and cleared every some time. but how is that different from <code>PG_referenced</code>, how can one be 1 and other be 0 or both share same value?</p>