mm, compaction: add the ultimate direct compaction priority

mm, compaction: add the ultimate direct compaction priority

During reclaim/compaction loop, it's desirable to get a final answer from
unsuccessful compaction so we can either fail the allocation or invoke the
OOM killer.  However, heuristics such as deferred compaction or pageblock
skip bits can cause compaction to skip parts or whole zones and lead to
premature OOM's, failures or excessive reclaim/compaction retries.

To remedy this, we introduce a new direct compaction priority called
COMPACT_PRIO_SYNC_FULL, which instructs direct compaction to:

- ignore deferred compaction status for a zone
- ignore pageblock skip hints
- ignore cached scanner positions and scan the whole zone

The new priority should get eventually picked up by should_compact_retry()
and this should improve success rates for costly allocations using
__GFP_REPEAT, such as hugetlbfs allocations, and reduce some corner-case
OOM's for non-costly allocations.

Link: http://lkml.kernel.org/r/20160810091226.6709-6-vbabka@suse.cz
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Rik van Riel <riel@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>