mm: compaction: prevent division-by-zero during user-requested compaction

Up until 3e7d3449 ("mm: vmscan: reclaim order-0 and use compaction instead
of lumpy reclaim"), compaction skipped calculating the fragmentation index
of a zone when compaction was explicitely requested through the procfs
knob.

However, when compaction_suitable was introduced, it did not come with an
extra check for order == -1, set on explicit compaction requests, and
passed this order on to the fragmentation index calculation, where it
overshifts the number of requested pages, leading to a division by zero.

This patch makes sure that order == -1 is recognized as the flag it is
rather than passing it along as valid order parameter.

[akpm@linux-foundation.org: add comment, per Mel]
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Mel Gorman <mel@csn.ul.ie>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/compaction.c

@@ -406,6 +406,10 @@ static int compact_finished(struct zone *zone,
 	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
+	 */
 	if (cc->order == -1)
 		return COMPACT_CONTINUE;
 
@@ -453,6 +457,13 @@ 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