The goal of this code is to make sure that used pages are marked as such.

This is accomplished by:

* When a process is descheduled - look in its mm for used pages - update 
corresponding page. (Done at most once per tick)

Pros:
* No need for extra elements in page
* Used page will be updated promptly

Cons:
* Context switch times will increase
* CPU cycle waste.

But:
* On my UP it does not affect performance that much :-)
- Lost at most 1,7 MB/s (-7%)
+ Gained in some cases (read, diff, dbench 4%)
[I thought it would become much worse - with other mm changes
 compensating]

-- 
Roger Larsson
Skelleftea
Sweden