mm, compaction: more reliably increase direct compaction priority

mm, compaction: more reliably increase direct compaction priority

During reclaim/compaction loop, compaction priority can be increased by
the should_compact_retry() function, but the current code is not optimal.
Priority is only increased when compaction_failed() is true, which means
that compaction has scanned the whole zone.  This may not happen even
after multiple attempts with a lower priority due to parallel activity, so
we might needlessly struggle on the lower priorities and possibly run out
of compaction retry attempts in the process.

After this patch we are guaranteed at least one attempt at the highest
compaction priority even if we exhaust all retries at the lower
priorities.

Link: http://lkml.kernel.org/r/20160906135258.18335-3-vbabka@suse.cz
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Michal Hocko <mhocko@kernel.org>
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>
Cc: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>