mm, numa: Retry failed page migrations

mm, numa: Retry failed page migrations

Keep track of how many NUMA page migrations succeeded and
failed (in a way that wants retrying later) per process.

If a lot of the page migrations of a process fail, unmap the
process pages some point later, so the migration can be tried
again at the next fault.

Signed-off-by: Rik van Riel <riel@redhat.com>
[ Fwd ported several times as the code changed, added some wrappers
  to aid compilability for certain CONFIG variants.
  Also need to note that the lack of atomics means we can go all funny
  but since its decaying stats it should recover eventually. If it
  goes funny too often we could look at improving this. ]
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/20120726135425.48820aae@cuia.bos.redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>