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/[email protected]
[[email protected]: use the MIN_COMPACT_PRIORITY alias]
  Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Vlastimil Babka <[email protected]>
Tested-by: Lorenzo Stoakes <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Joonsoo Kim <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Rik van Riel <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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