[PATCH 1/4] mm, vmscan: Have kswapd reclaim from all zones if reclaiming and buffer_heads_over_limit -fix

[Mel Gorman <mgorman@techsingularity.net>]

Johannes reported that the comment about buffer_heads_over_limit in
balance_pgdat only made sense in the context of the patch. This patch
clarifies the reasoning and how it applies to 32 and 64 bit systems.

This is a fix to the mmotm patch
mm-vmscan-have-kswapd-reclaim-from-all-zones-if-reclaiming-and-buffer_heads_over_limit.patch

Suggested-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
---
 mm/vmscan.c | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index d079210d46ee..21eae17ee730 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -3131,12 +3131,13 @@ static int balance_pgdat(pg_data_t *pgdat, int order, int classzone_idx)
 
 		/*
 		 * If the number of buffer_heads exceeds the maximum allowed
-		 * then consider reclaiming from all zones. This is not
-		 * specific to highmem which may not exist but it is it is
-		 * expected that buffer_heads are stripped in writeback.
-		 * Reclaim may still not go ahead if all eligible zones
-		 * for the original allocation request are balanced to
-		 * avoid excessive reclaim from kswapd.
+		 * then consider reclaiming from all zones. This has a dual
+		 * purpose -- on 64-bit systems it is expected that
+		 * buffer_heads are stripped during active rotation. On 32-bit
+		 * systems, highmem pages can pin lowmem memory and shrinking
+		 * buffers can relieve lowmem pressure. Reclaim may still not
+		 * go ahead if all eligible zones for the original allocation
+		 * request are balanced to avoid excessive reclaim from kswapd.
 		 */
 		if (buffer_heads_over_limit) {
 			for (i = MAX_NR_ZONES - 1; i >= 0; i--) {
-- 
2.6.4

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