During an AIM7 run on a 16GB system, fork started failing around 32000
threads, despite the system having plenty of free swap and 15GB of
pageable memory.  This was on x86-64, so 8k stacks.

If a higher order allocation fails, we can either:
- keep evicting pages off the end of the LRUs and hope that
  we eventually create a contiguous region; this is somewhat
  unlikely if the system is under enough stress by new
  allocations
- after trying normal eviction for a bit, use lumpy reclaim

This patch switches the system to lumpy reclaim if the VM is having
trouble freeing enough pages, using the same threshold for detection as
used by pageout congestion wait.
Signed-off-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>