[PATCH -V10 8/9] mm/vmscan: never demote for memcg reclaim

[Huang Ying]

From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>

Global reclaim aims to reduce the amount of memory used on
a given node or set of nodes.  Migrating pages to another
node serves this purpose.

memcg reclaim is different.  Its goal is to reduce the
total memory consumption of the entire memcg, across all
nodes.  Migration does not assist memcg reclaim because
it just moves page contents between nodes rather than
actually reducing memory consumption.

Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
Suggested-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
Reviewed-by: Yang Shi <shy828301@xxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Wei Xu <weixugc@xxxxxxxxxx>
Cc: osalvador <osalvador@xxxxxxx>
Cc: Zi Yan <ziy@xxxxxxxxxx>
Cc: David Rientjes <rientjes@xxxxxxxxxx>
Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
Cc: David Hildenbrand <david@xxxxxxxxxx>

--
changes from 20210618:
 * Avoid to scan anon lists for demotion for cgroup reclaim.
---
 mm/vmscan.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 0f9be998230f..b697f1a6108c 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -521,8 +521,13 @@ static long add_nr_deferred(long nr, struct shrinker *shrinker,
 
 static bool can_demote_anon_pages(int nid, struct scan_control *sc)
 {
-	if (sc && sc->no_demotion)
-		return false;
+	if (sc) {
+		if (sc->no_demotion)
+			return false;
+		/* It is pointless to do demotion in memcg reclaim */
+		if (cgroup_reclaim(sc))
+			return false;
+	}
 	if (next_demotion_node(nid) == NUMA_NO_NODE)
 		return false;
 
-- 
2.30.2