mm/vmscan.c: use DIV_ROUND_UP for calculation of zone's balance_gap and correct comments.

Currently, we use (zone->managed_pages + KSWAPD_ZONE_BALANCE_GAP_RATIO-1)
/ KSWAPD_ZONE_BALANCE_GAP_RATIO to avoid a zero gap value.  It's better to
use DIV_ROUND_UP macro for neater code and clear meaning.

Besides, the gap value is calculated against the per-zone "managed pages",
not "present pages".  This patch also corrects the comment and do some
rephrasing.

Signed-off-by: Jianyu Zhan <nasa4836@gmail.com>
Acked-by: Rik van Riel <riel@redhat.com>
Acked-by: Rafael Aquini <aquini@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 4 insertions, 6 deletions

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 494cd632178c..cc29fca8d989 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2295,9 +2295,8 @@ static inline bool compaction_ready(struct zone *zone, struct scan_control *sc)
          * there is a buffer of free pages available to give compaction
          * a reasonable chance of completing and allocating the page
          */
-        balance_gap = min(low_wmark_pages(zone),
-                (zone->managed_pages + KSWAPD_ZONE_BALANCE_GAP_RATIO-1) /
-                        KSWAPD_ZONE_BALANCE_GAP_RATIO);
+        balance_gap = min(low_wmark_pages(zone), DIV_ROUND_UP(
+                        zone->managed_pages, KSWAPD_ZONE_BALANCE_GAP_RATIO));
         watermark = high_wmark_pages(zone) + balance_gap + (2UL << sc->order);
         watermark_ok = zone_watermark_ok_safe(zone, 0, watermark, 0, 0);
 
@@ -2949,9 +2948,8 @@ static bool kswapd_shrink_zone(struct zone *zone,
          * high wmark plus a "gap" where the gap is either the low
          * watermark or 1% of the zone, whichever is smaller.
          */
-        balance_gap = min(low_wmark_pages(zone),
-                (zone->managed_pages + KSWAPD_ZONE_BALANCE_GAP_RATIO-1) /
-                KSWAPD_ZONE_BALANCE_GAP_RATIO);
+        balance_gap = min(low_wmark_pages(zone), DIV_ROUND_UP(
+                        zone->managed_pages, KSWAPD_ZONE_BALANCE_GAP_RATIO));
 
         /*
          * If there is no low memory pressure or the zone is balanced then no