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>

diff --git a/mm/compaction.c b/mm/compaction.c
index 6d592a0..8be430b 100644 (file)
--- 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;
 
@@ -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