[PATCH 1/2] check the return value of soft_limit reclaim

[Ying Han <yinghan@google.com>]

In the global background reclaim, we do soft reclaim before scanning the
per-zone LRU. However, the return value is ignored. This patch adds the logic
where no per-zone reclaim happens if the soft reclaim raise the free pages
above the zone's high_wmark.

I did notice a similar check exists but instead leaving a "gap" above the
high_wmark(the code right after my change in vmscan.c). There are discussions
on whether or not removing the "gap" which intends to balance pressures across
zones over time. Without fully understand the logic behind, I didn't try to
merge them into one, but instead adding the condition only for memcg users
who care a lot on memory isolation.

Signed-off-by: Ying Han <yinghan@google.com>
---
 mm/vmscan.c |   16 +++++++++++++++-
 1 files changed, 15 insertions(+), 1 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 060e4c1..e4601c5 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2320,6 +2320,7 @@ static unsigned long balance_pgdat(pg_data_t *pgdat, int order,
 	int end_zone = 0;	/* Inclusive.  0 = ZONE_DMA */
 	unsigned long total_scanned;
 	struct reclaim_state *reclaim_state = current->reclaim_state;
+	unsigned long nr_soft_reclaimed;
 	struct scan_control sc = {
 		.gfp_mask = GFP_KERNEL,
 		.may_unmap = 1,
@@ -2413,7 +2414,20 @@ loop_again:
 			 * Call soft limit reclaim before calling shrink_zone.
 			 * For now we ignore the return value
 			 */
-			mem_cgroup_soft_limit_reclaim(zone, order, sc.gfp_mask);
+			nr_soft_reclaimed = mem_cgroup_soft_limit_reclaim(zone,
+							order, sc.gfp_mask);
+
+			/*
+			 * Check the watermark after the soft limit reclaim. If
+			 * the free pages is above the watermark, no need to
+			 * proceed to the zone reclaim.
+			 */
+			if (nr_soft_reclaimed && zone_watermark_ok_safe(zone,
+					order, high_wmark_pages(zone),
+					end_zone, 0)) {
+				__inc_zone_state(zone, NR_SKIP_RECLAIM_GLOBAL);
+				continue;
+			}
 
 			/*
 			 * We put equal pressure on every zone, unless
-- 
1.7.3.1

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