From: Nick Piggin <nickpiggin@yahoo.com.au>

If the zone has a very small number of inactive pages, local variable
`ratio' can be huge and we do way too much scanning.  So much so that Ingo
hit an NMI watchdog expiry, although that was because the zone would have a
had a single refcount-zero page in it, and that logic recently got fixed up
via get_page_testone().

Nick's patch simply puts a sane-looking upper bound on the number of pages
which we'll scan in this round.


It fixes another failure case: if the inactive list becomes very small
compared to the size of the active list, active list scanning (and therefore
inactive list refilling) also becomes small.

This patch causes inactive list scanning to be keyed off the size of the
active+inactive lists.  It has the plus of hiding active and inactive
balancing implementation from the higher level scanning code.  It will
slightly change other aspects of scanning behaviour, but probably not
significantly.