[PATCH] vmscan: skip freeing memory from zones with lots free

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

Skip freeing memory from zones that already have lots of free memory.
If one memory zone has harder to free memory, we want to avoid freeing
excessive amounts of memory from other zones, if only because pageout
IO from the other zones can slow down page freeing from the problem zone.

This is similar to the check already done by kswapd in balance_pgdat().

Signed-off-by: Rik van Riel <riel@redhat.com>
---
Kosaki-san, this should address point (3) from your list.

 mm/vmscan.c |    3 +++
 1 file changed, 3 insertions(+)

Index: linux-2.6.28-rc5/mm/vmscan.c
===================================================================
--- linux-2.6.28-rc5.orig/mm/vmscan.c	2008-11-28 05:53:56.000000000 -0500
+++ linux-2.6.28-rc5/mm/vmscan.c	2008-11-28 06:05:29.000000000 -0500
@@ -1510,6 +1510,9 @@ static unsigned long shrink_zones(int pr
 			if (zone_is_all_unreclaimable(zone) &&
 						priority != DEF_PRIORITY)
 				continue;	/* Let kswapd poll it */
+			if (zone_watermark_ok(zone, sc->order,
+					4*zone->pages_high, high_zoneidx, 0))
+				continue;	/* Lots free already */
 			sc->all_unreclaimable = 0;
 		} else {
 			/*

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