vmscan: avoid subtraction of unsigned types

'slab_reclaimable' and 'nr_pages' are unsigned.  Subtraction is unsafe
because negative results would be misinterpreted.

Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Cc: Rik van Riel <riel@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 8 insertions, 7 deletions

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 1c3d960de9d2..1b4e4a597caa 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2600,7 +2600,7 @@ static int __zone_reclaim(struct zone *zone, gfp_t gfp_mask, unsigned int order)
                 .swappiness = vm_swappiness,
                 .order = order,
         };
-        unsigned long slab_reclaimable;
+        unsigned long nr_slab_pages0, nr_slab_pages1;
 
         cond_resched();
         /*
@@ -2625,8 +2625,8 @@ static int __zone_reclaim(struct zone *zone, gfp_t gfp_mask, unsigned int order)
                 } while (priority >= 0 && sc.nr_reclaimed < nr_pages);
         }
 
-        slab_reclaimable = zone_page_state(zone, NR_SLAB_RECLAIMABLE);
-        if (slab_reclaimable > zone->min_slab_pages) {
+        nr_slab_pages0 = zone_page_state(zone, NR_SLAB_RECLAIMABLE);
+        if (nr_slab_pages0 > zone->min_slab_pages) {
                 /*
                  * shrink_slab() does not currently allow us to determine how
                  * many pages were freed in this zone. So we take the current
@@ -2638,16 +2638,17 @@ static int __zone_reclaim(struct zone *zone, gfp_t gfp_mask, unsigned int order)
                  * take a long time.
                  */
                 while (shrink_slab(sc.nr_scanned, gfp_mask, order) &&
-                        zone_page_state(zone, NR_SLAB_RECLAIMABLE) >
-                                slab_reclaimable - nr_pages)
+                       (zone_page_state(zone, NR_SLAB_RECLAIMABLE) + nr_pages >
+                                nr_slab_pages0))
                         ;
 
                 /*
                  * Update nr_reclaimed by the number of slab pages we
                  * reclaimed from this zone.
                  */
-                sc.nr_reclaimed += slab_reclaimable -
-                        zone_page_state(zone, NR_SLAB_RECLAIMABLE);
+                nr_slab_pages1 = zone_page_state(zone, NR_SLAB_RECLAIMABLE);
+                if (nr_slab_pages1 < nr_slab_pages0)
+                        sc.nr_reclaimed += nr_slab_pages0 - nr_slab_pages1;
         }
 
         p->reclaim_state = NULL;