mm: make unreserve highatomic functions reliable

Currently, unreserve_highatomic_pageblock bails out if it found highatomic
pageblock regardless of really moving free pages from the one so that it
could mitigate unreserve logic's goal which saves OOM of a process.

This patch makes unreserve functions bail out only if it moves some pages
out of !highatomic free list to avoid such false positive.

Another potential problem is that by race between page freeing and reserve
highatomic function, pages could be in highatomic free list even though
the pageblock is !high atomic migratetype.  In that case,
unreserve_highatomic_pageblock can be void if count of highatomic reserve
is less than pageblock_nr_pages.  We could solve it simply via draining
all of reserved pages before the OOM.  It would have a safeguard role to
exhuast reserved pages before converging to OOM.

Link: http://lkml.kernel.org/r/1476259429-18279-5-git-send-email-minchan@kernel.org
Signed-off-by: Minchan Kim <minchan@kernel.org>
Signed-off-by: Michal Hocko <mhocko@suse.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Sangseok Lee <sangseok.lee@lge.com>
Cc: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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