[PATCH -v4 4/4] mm/mglru: fix anon-only reclaim evicting file pages when swappiness=max

[Ridong]

From: Ridong Chen <chenridong@xxxxxxxxxx>

The previous patch fixed this issue for the traditional LRU. The same
problem exists in MGLRU [1]: when swappiness=max (SWAPPINESS_ANON_ONLY)
is set, reclaim is expected to evict anonymous pages exclusively, but
file pages can still be reclaimed when anonymous pages cannot be
reclaimed (e.g. no swap and no demotion target).

Fix it the same way as the traditional LRU: keep returning
SWAPPINESS_ANON_ONLY in get_swappiness(), and return 0 from
get_nr_to_scan() when SWAPPINESS_ANON_ONLY is set but anon pages cannot
be reclaimed. Since get_nr_to_scan() decides how much MGLRU scans,
returning 0 skips the scan entirely and avoids the useless scan work
when there is nothing eligible to reclaim.

The test result:
Before fix:

  # cat /sys/kernel/mm/lru_gen/enabled
  0x0007
  # cat memory.stat
  anon 204800
  file 67108864
  ...
  pgsteal_proactive 0
  pgscan_proactive 0

  # echo "64M swappiness=max" > memory.reclaim
  # cat memory.stat
  anon 208896
  file 0
  ...
  pgsteal_proactive 16384
  pgscan_proactive 16384

After fix:

  # cat memory.stat
  anon 188416
  file 67215360
  kernel 1970176
  ...
  pgsteal_proactive 0
  pgscan_proactive 0

  # echo "64M swappiness=max" > memory.reclaim
  -bash: echo: write error: Resource temporarily unavailable
  # cat memory.stat
  anon 204800
  file 67215360
  ...
  pgsteal_proactive 0
  pgscan_proactive 0

[1] https://sashiko.dev/#/patchset/20260717113300.214717-1-ridong.chen@xxxxxxxxx

Fixes: 68a1436bde00 ("mm: add swappiness=max arg to memory.reclaim for only anon reclaim")
Signed-off-by: Ridong Chen <chenridong@xxxxxxxxxx>
---
 mm/vmscan.c | 20 +++++++++++++++++++-
 1 file changed, 19 insertions(+), 1 deletion(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 859655fcf60d..254827544461 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2708,6 +2708,10 @@ static int get_swappiness(struct lruvec *lruvec, struct scan_control *sc)
 {
 	struct mem_cgroup *memcg = lruvec_memcg(lruvec);
 	struct pglist_data *pgdat = lruvec_pgdat(lruvec);
+	int swappiness = sc_swappiness(sc, memcg);
+
+	if (swappiness == SWAPPINESS_ANON_ONLY)
+		return swappiness;
 
 	if (!sc->may_swap)
 		return 0;
@@ -2716,7 +2720,7 @@ static int get_swappiness(struct lruvec *lruvec, struct scan_control *sc)
 	    mem_cgroup_get_nr_swap_pages(memcg) < MIN_LRU_BATCH)
 		return 0;
 
-	return sc_swappiness(sc, memcg);
+	return swappiness;
 }
 
 static int get_nr_gens(struct lruvec *lruvec, int type)
@@ -4921,6 +4925,20 @@ static long get_nr_to_scan(struct lruvec *lruvec, struct scan_control *sc,
 			   struct mem_cgroup *memcg, int swappiness)
 {
 	unsigned long nr_to_scan, evictable;
+	struct pglist_data *pgdat = lruvec_pgdat(lruvec);
+
+	/*
+	 * Proactive reclaim initiated by userspace for anonymous memory only.
+	 * SWAPPINESS_ANON_ONLY is set only on the proactive reclaim path, so
+	 * warn if it shows up elsewhere. When anon cannot be reclaimed (e.g.
+	 * no swap), return 0 to skip the scan entirely, avoiding useless scan
+	 * work when there is nothing eligible to reclaim.
+	 */
+	if (swappiness == SWAPPINESS_ANON_ONLY) {
+		WARN_ON_ONCE(!sc->proactive);
+		if (!can_reclaim_anon_pages(memcg, pgdat->node_id, sc))
+			return 0;
+	}
 
 	evictable = lruvec_evictable_size(lruvec, swappiness);
 
-- 
2.34.1