[RFC][PATCH -mm] vmscan: fix swapout on sequential IO

[Rik van Riel <riel@surriel.com>]

Only force the scanning of every LRU list if we could not easily
find a page to evict.  This preserves the balancing done by
get_scan_ratio(), while ensuring the VM will make progress
when there is serious memory pressure.

Signed-off-by: Rik van Riel <riel@redhat.com>
---
This should fix the "dd if=/dev/sda of=/dev/null causes swapout"
problem that has been seen with the new split LRU VM.

 mm/vmscan.c |   17 ++++++++++-------
 1 file changed, 10 insertions(+), 7 deletions(-)

Index: linux-2.6.26-rc8-mm1/mm/vmscan.c
===================================================================
--- linux-2.6.26-rc8-mm1.orig/mm/vmscan.c	2008-07-23 14:19:09.000000000 -0400
+++ linux-2.6.26-rc8-mm1/mm/vmscan.c	2008-07-23 14:36:26.000000000 -0400
@@ -1447,24 +1447,27 @@ static unsigned long shrink_zone(int pri
 	unsigned long nr_to_scan;
 	unsigned long nr_reclaimed = 0;
 	unsigned long percent[2];	/* anon @ 0; file @ 1 */
+	unsigned int force_scan = 0;
 	enum lru_list l;
 
+	/*
+         * If we do not immediately find pages to evict, put some
+         * pressure on every LRU list to guarantee progress.
+         */
+	if (priority < DEF_PRIORITY - 2)
+		force_scan = 1;
+
 	get_scan_ratio(zone, sc, percent);
 
 	for_each_evictable_lru(l) {
 		if (scan_global_lru(sc)) {
 			int file = is_file_lru(l);
-			int scan;
-			/*
-			 * Add one to nr_to_scan just to make sure that the
-			 * kernel will slowly sift through each list.
-			 */
-			scan = zone_page_state(zone, NR_LRU_BASE + l);
+			int scan = zone_page_state(zone, NR_LRU_BASE + l);
 			if (priority) {
 				scan >>= priority;
 				scan = (scan * percent[file]) / 100;
 			}
-			zone->lru[l].nr_scan += scan + 1;
+			zone->lru[l].nr_scan += scan + force_scan;
 			nr[l] = zone->lru[l].nr_scan;
 			if (nr[l] >= sc->swap_cluster_max)
 				zone->lru[l].nr_scan = 0;

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