should_reclaim_retry will give up retries for higher order allocations
if none of the eligible zones has any requested or higher order pages
available even if we pass the watermak check for order-0.  This is done
because there is no guarantee that the reclaimable and currently free
pages will form the required order.

This can, however, lead to situations where the high-order request (e.g.
order-2 required for the stack allocation during fork) will trigger OOM
too early - e.g.  after the first reclaim/compaction round.  Such a
system would have to be highly fragmented and there is no guarantee
further reclaim/compaction attempts would help but at least make sure
that the compaction was active before we go OOM and keep retrying even
if should_reclaim_retry tells us to oom if

	- the last compaction round backed off or
	- we haven't completed at least MAX_COMPACT_RETRIES active
	  compaction rounds.

The first rule ensures that the very last attempt for compaction was not
ignored while the second guarantees that the compaction has done some
work.  Multiple retries might be needed to prevent occasional pigggy
backing of other contexts to steal the compacted pages before the
current context manages to retry to allocate them.

compaction_failed() is taken as a final word from the compaction that
the retry doesn't make much sense.  We have to be careful though because
the first compaction round is MIGRATE_ASYNC which is rather weak as it
ignores pages under writeback and gives up too easily in other
situations.  We therefore have to make sure that MIGRATE_SYNC_LIGHT mode
has been used before we give up.  With this logic in place we do not
have to increase the migration mode unconditionally and rather do it
only if the compaction failed for the weaker mode.  A nice side effect
is that the stronger migration mode is used only when really needed so
this has a potential of smaller latencies in some cases.

Please note that the compaction doesn't tell us much about how
successful it was when returning compaction_made_progress so we just
have to blindly trust that another retry is worthwhile and cap the
number to something reasonable to guarantee a convergence.

If the given number of successful retries is not sufficient for a
reasonable workloads we should focus on the collected compaction
tracepoints data and try to address the issue in the compaction code.
If this is not feasible we can increase the retries limit.

[mhocko@suse.com: fix warning]
  Link: http://lkml.kernel.org/r/20160512061636.GA4200@dhcp22.suse.czSigned-off-by: Michal Hocko <mhocko@suse.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Hillf Danton <hillf.zj@alibaba-inc.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Joonsoo Kim <js1304@gmail.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: Vladimir Davydov <vdavydov@virtuozzo.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>