While caching the last used vma already does a nice job avoiding
having to iterate the rbtree in find_vma, we can improve. After
studying the hit rate on a load of workloads and environments,
it was seen that it was around 45-50% - constant for a standard
desktop system (gnome3 + evolution + firefox + a few xterms),
and multiple java related workloads (including Hadoop/terasort),
and aim7, which indicates it's better than the 35% value documented
in the code.
By also caching the largest vma, that is, the one that contains
most addresses, there is a steady 10-15% hit rate gain, putting
it above the 60% region. This improvement comes at a very low
overhead for a miss. Furthermore, systems with !CONFIG_MMU keep
the current logic.