mm: compaction: fix special case -1 order checks

Commit 56de7263 ("mm: compaction: direct compact when a high-order
allocation fails") introduced a check for cc->order == -1 in
compact_finished.  We should continue compacting in that case because
the request came from userspace and there is no particular order to
compact for.  Similar check has been added by 82478fb7 (mm: compaction:
prevent division-by-zero during user-requested compaction) for
compaction_suitable.

The check is, however, done after zone_watermark_ok which uses order as a
right hand argument for shifts.  Not only watermark check is pointless if
we can break out without it but it also uses 1 << -1 which is not well
defined (at least from C standard).  Let's move the -1 check above
zone_watermark_ok.

[minchan.kim@gmail.com> - caught compaction_suitable]
Signed-off-by: Michal Hocko <mhocko@suse.cz>
Cc: Mel Gorman <mgorman@suse.de>
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>
Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hioryu@jp.fujitsu.com>
Acked-by: Mel Gorman <mgorman@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/compaction.c

@@ -420,13 +420,6 @@ static int compact_finished(struct zone *zone,
 	if (cc->free_pfn <= cc->migrate_pfn)
 		return COMPACT_COMPLETE;
 
-	/* Compaction run is not finished if the watermark is not met */
-	watermark = low_wmark_pages(zone);
-	watermark += (1 << cc->order);
-
-	if (!zone_watermark_ok(zone, cc->order, watermark, 0, 0))
-		return COMPACT_CONTINUE;
-
 	/*
 	 * order == -1 is expected when compacting via
 	 * /proc/sys/vm/compact_memory
@@ -434,6 +427,13 @@ static int compact_finished(struct zone *zone,
 	if (cc->order == -1)
 		return COMPACT_CONTINUE;
 
+	/* Compaction run is not finished if the watermark is not met */
+	watermark = low_wmark_pages(zone);
+	watermark += (1 << cc->order);
+
+	if (!zone_watermark_ok(zone, cc->order, watermark, 0, 0))
+		return COMPACT_CONTINUE;
+
 	/* Direct compactor: Is a suitable page free? */
 	for (order = cc->order; order < MAX_ORDER; order++) {
 		/* Job done if page is free of the right migratetype */
@@ -460,6 +460,13 @@ unsigned long compaction_suitable(struct zone *zone, int order)
 	int fragindex;
 	unsigned long watermark;
 
+	/*
+	 * order == -1 is expected when compacting via
+	 * /proc/sys/vm/compact_memory
+	 */
+	if (order == -1)
+		return COMPACT_CONTINUE;
+
 	/*
 	 * Watermarks for order-0 must be met for compaction. Note the 2UL.
 	 * This is because during migration, copies of pages need to be
@@ -469,13 +476,6 @@ unsigned long compaction_suitable(struct zone *zone, int order)
 	if (!zone_watermark_ok(zone, 0, watermark, 0, 0))
 		return COMPACT_SKIPPED;
 
-	/*
-	 * order == -1 is expected when compacting via
-	 * /proc/sys/vm/compact_memory
-	 */
-	if (order == -1)
-		return COMPACT_CONTINUE;
-
 	/*
 	 * fragmentation index determines if allocation failures are due to
 	 * low memory or external fragmentation