The patch addresses some search complexity failures which occur when
there is a large amount of dirty data on the inactive list.

Normally we attempt to write out those pages and then move them to the
head of the inactive list.  But this goes against page aging, and means
that the page has to traverse the entire list again before it can be
reclaimed.

But the VM really wants to reclaim that page - it has reached the tail
of the LRU.

So what we do in this patch is to mark the page as needing reclamation,
and then start I/O.  In the IO completion handler we check to see if
the page is still probably reclaimable and if so, move it to the tail of
the inactive list, where it can be reclaimed immediately.

Under really heavy swap-intensive loads this increases the page reclaim
efficiency (pages reclaimed/pages scanned) from 10% to 25%.  Which is
OK for that sort of load.  Not great, but OK.

This code path takes the LRU lock once per page.  I didn't bother
playing games with batching up the locking work - it's a rare code
path, and the machine has plenty of CPU to spare when this is
happening.