[RFC][PATCH 6/8] mem_notify v5: (optional) fixed incorrect shrink_zone

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

on X86, ZONE_DMA is very very small.
It is often no used at all. 

Unfortunately, 
when NR_ACTIVE==0, NR_INACTIVE==0, shrink_zone() try to reclaim 1 page.
because

    zone->nr_scan_active +=
        (zone_page_state(zone, NR_ACTIVE) >> priority) + 1;
                                                        ^^^^^

it cause unnecessary low memory notify ;-)


ChangeLog
	v5: new

---
 mm/vmscan.c |   21 ++++++++++++++++-----
 1 file changed, 16 insertions(+), 5 deletions(-)

Index: b/mm/vmscan.c
===================================================================
--- a/mm/vmscan.c	2008-01-18 14:18:27.000000000 +0900
+++ b/mm/vmscan.c	2008-01-18 14:49:06.000000000 +0900
@@ -948,7 +948,7 @@ static inline void note_zone_scanning_pr
 
 static inline int zone_is_near_oom(struct zone *zone)
 {
-	return zone->pages_scanned >= (zone_page_state(zone, NR_ACTIVE)
+	return zone->pages_scanned > (zone_page_state(zone, NR_ACTIVE)
 				+ zone_page_state(zone, NR_INACTIVE))*3;
 }
 
@@ -1214,18 +1214,29 @@ static unsigned long shrink_zone(int pri
 	unsigned long nr_inactive;
 	unsigned long nr_to_scan;
 	unsigned long nr_reclaimed = 0;
+	unsigned long tmp;
+	unsigned long zone_active;
+	unsigned long zone_inactive;
 
 	if (scan_global_lru(sc)) {
 		/*
 		 * Add one to nr_to_scan just to make sure that the kernel
 		 * will slowly sift through the active list.
 		 */
-		zone->nr_scan_active +=
-			(zone_page_state(zone, NR_ACTIVE) >> priority) + 1;
+		zone_active = zone_page_state(zone, NR_ACTIVE);
+		tmp = (zone_active >> priority) + 1;
+		if (unlikely(tmp > zone_active))
+			tmp = zone_active;
+		zone->nr_scan_active += tmp;
 		nr_active = zone->nr_scan_active;
-		zone->nr_scan_inactive +=
-			(zone_page_state(zone, NR_INACTIVE) >> priority) + 1;
+
+		zone_inactive = zone_page_state(zone, NR_INACTIVE);
+		tmp = (zone_inactive >> priority) + 1;
+		if (unlikely(tmp > zone_inactive))
+			tmp = zone_inactive;
+		zone->nr_scan_inactive += tmp;
 		nr_inactive = zone->nr_scan_inactive;
+
 		if (nr_inactive >= sc->swap_cluster_max)
 			zone->nr_scan_inactive = 0;
 		else


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