mm: compaction: fix special case -1 order checks

Commit 56de7263fcf3 ("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>

1 files changed, 14 insertions, 14 deletions

diff --git a/mm/compaction.c b/mm/compaction.c
index 021a2960ef9e..94bdbe1f7caf 100644
--- a/mm/compaction.c
+++ b/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 */
@@ -461,6 +461,13 @@ unsigned long compaction_suitable(struct zone *zone, int order)
         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
          * allocated and for a short time, the footprint is higher
@@ -470,13 +477,6 @@ 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
          *