[PATCH] mm: limit lowmem_reserve

[Con Kolivas <kernel@kolivas.org>]

It is possible with a low enough lowmem_reserve ratio to make
zone_watermark_ok always fail if the lower_zone is small enough.
Impose a lower limit on the ratio to only allow 1/4 of the lower_zone
size to be set as lowmem_reserve. This limit is hit in ZONE_DMA by changing
the default vmsplit on i386 even without changing the default sysctl values.

Signed-off-by: Con Kolivas <kernel@kolivas.org>

---
 mm/page_alloc.c |   24 +++++++++++++++++++++---
 1 files changed, 21 insertions(+), 3 deletions(-)

Index: linux-2.6.17-rc1-mm1/mm/page_alloc.c
===================================================================
--- linux-2.6.17-rc1-mm1.orig/mm/page_alloc.c	2006-04-06 10:32:31.000000000 +1000
+++ linux-2.6.17-rc1-mm1/mm/page_alloc.c	2006-04-06 11:09:17.000000000 +1000
@@ -2566,14 +2566,32 @@ static void setup_per_zone_lowmem_reserv
 			zone->lowmem_reserve[j] = 0;
 
 			for (idx = j-1; idx >= 0; idx--) {
+				unsigned long max_reserve;
+				unsigned long reserve;
 				struct zone *lower_zone;
 
+				lower_zone = pgdat->node_zones + idx;
+				/*
+				 * Put an upper limit on the reserve at 1/4
+				 * the lower_zone size. This prevents large
+				 * zone size differences such as 3G VMSPLIT
+				 * or low sysctl values from making
+				 * zone_watermark_ok always fail. This
+				 * enforces a lower limit on the reserve_ratio
+				 */
+				max_reserve = lower_zone->present_pages / 4;
+
 				if (sysctl_lowmem_reserve_ratio[idx] < 1)
 					sysctl_lowmem_reserve_ratio[idx] = 1;
-
-				lower_zone = pgdat->node_zones + idx;
-				lower_zone->lowmem_reserve[j] = present_pages /
+				reserve = present_pages /
 					sysctl_lowmem_reserve_ratio[idx];
+				if (reserve > max_reserve) {
+					reserve = max_reserve;
+					sysctl_lowmem_reserve_ratio[idx] =
+						present_pages / max_reserve;
+				}
+
+				lower_zone->lowmem_reserve[j] = reserve;
 				present_pages += lower_zone->present_pages;
 			}
 		}

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