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@xxxxxxxxxxx>
Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
---
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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