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SubjectRe: [PATCH 12/34] mm: vmscan: do not reclaim from kswapd if there is any eligible zone
On Tue, Jul 12, 2016 at 10:29:09AM -0400, Johannes Weiner wrote:
> > + /*
> > + * If the number of buffer_heads in the machine exceeds the
> > + * maximum allowed level then reclaim from all zones. This is
> > + * not specific to highmem as highmem may not exist but it is
> > + * it is expected that buffer_heads are stripped in writeback.
>
> The mention of highmem in this comment make only sense within the
> context of this diff; it'll be pretty confusing in the standalone
> code.
>
> Also, double "it is" :)

Is this any better?

Note that it's marked as a fix to a later patch to reduce collisions in
mmotm. It's not a bisection risk so I saw little need to cause
unnecessary conflicts for Andrew.

---8<---
mm, vmscan: Have kswapd reclaim from all zones if reclaiming and buffer_heads_over_limit -fix

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>

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--) {
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