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

Up until 3e7d344 ("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>

1 files changed, 11 insertions, 0 deletions

diff --git a/mm/compaction.c b/mm/compaction.c
index 6d592a021072..8be430b812de 100644
--- a/mm/compaction.c
+++ b/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;
 
@@ -454,6 +458,13 @@ unsigned long compaction_suitable(struct zone *zone, int order)
                 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
          *