/proc/sys/vm/swappiness controls the VM's tendency to unmap pages and to
swap things out.

100 -> basically current 2.5 behaviour
0 -> not very swappy at all

The mechanism which is used to control swappiness is: to be reluctant
to bring mapped pages onto the inactive list.  Prefer to reclaim
pagecache instead.

The control for that mechanism is as follows:

- If there is a large amount of mapped memory in the machine, we
  prefer to bring mapped pages onto the inactive list.

- If page reclaim is under distress (more scanning is happening) then
  prefer to bring mapped pages onto the inactive list.  This is
  basically the 2.4 algorithm, really.

- If the /proc/sys/vm/swappiness control is high then prefer to bring
  mapped pages onto the inactive list.

The implementation is simple: calculate the above three things as
percentages and add them up.  If that's over 100% then start reclaiming
mapped pages.

The `proportion of mapped memory' is downgraded so that we don't swap
just because a lot of memory is mapped into pagetables - we still need
some VM distress before starting to swap that memory out.

For a while I was adding a little bias so that we prefer to unmap
file-backed memory before swapping out anon memory.  Because usually
file backed memory can be evicted and reestablished with one I/O, not
two.  It was unmapping executable text too easily, so here I just treat
them equally.