[PATCH 1/3] mm/vmscan: fix anon-only reclaim evicting file pages when swappiness=max

[Ridong <ridong.chen@linux.dev>]

From: Ridong Chen <chenridong@xiaomi.com>

As Qi mentioned [1], when swappiness=max (SWAPPINESS_ANON_ONLY) is set,
the reclaim logic is expected to reclaim anonymous pages exclusively.
However, due to the current ordering of checks in get_scan_count(),
file pages may still be evicted if can_reclaim_anon_pages() returns
false, which contradicts the semantics of SWAPPINESS_ANON_ONLY.

Reproducer in a cgroup holding 64M of file cache, with no swap configured:

  Before (file cache is wrongly evicted):
    # cat memory.stat
    anon 196608
    file 67178496
    pgscan_proactive 0
    # echo "64M swappiness=max" > memory.reclaim
    # cat memory.stat
    anon 208896
    file 4096                 <- page cache evicted
    pgsteal_proactive 16400
    pgscan_proactive 16400

  After (file cache is left intact):
    # cat memory.stat
    anon 200704
    file 67178496
    pgscan_proactive 0
    # echo "64M swappiness=max" > memory.reclaim
    -bash: echo: write error: Resource temporarily unavailable
    # cat memory.stat
    anon 208896
    file 67178496             <- page cache untouched
    pgsteal_proactive 0
    pgscan_proactive 0

Fix this by bailing out early when SWAPPINESS_ANON_ONLY is set and no
anonymous pages are reclaimable, before falling back to file reclaim.

[1] https://lore.kernel.org/cgroups/7ddf3eee-5fe2-45f7-8614-c8936a039e04@linux.dev/

Fixes: 68a1436bde00 ("mm: add swappiness=max arg to memory.reclaim for only anon reclaim")
Suggested-by: Qi Zheng <qi.zheng@linux.dev>
Signed-off-by: Ridong Chen <chenridong@xiaomi.com>
---
 mm/vmscan.c | 18 +++++++++++-------
 1 file changed, 11 insertions(+), 7 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 4357a44ee876..098adc599720 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2492,6 +2492,17 @@ static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc,
 	enum scan_balance scan_balance;
 	enum lru_list lru;
 
+	/* Proactive reclaim initiated by userspace for anonymous memory only */
+	if (swappiness == SWAPPINESS_ANON_ONLY) {
+		WARN_ON_ONCE(!sc->proactive);
+		if (!can_reclaim_anon_pages(memcg, pgdat->node_id, sc)) {
+			memset(nr, 0, sizeof(*nr) * NR_LRU_LISTS);
+			return;
+		}
+		scan_balance = SCAN_ANON;
+		goto out;
+	}
+
 	/* If we have no swap space, do not bother scanning anon folios. */
 	if (!sc->may_swap || !can_reclaim_anon_pages(memcg, pgdat->node_id, sc)) {
 		scan_balance = SCAN_FILE;
@@ -2510,13 +2521,6 @@ static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc,
 		goto out;
 	}
 
-	/* Proactive reclaim initiated by userspace for anonymous memory only */
-	if (swappiness == SWAPPINESS_ANON_ONLY) {
-		WARN_ON_ONCE(!sc->proactive);
-		scan_balance = SCAN_ANON;
-		goto out;
-	}
-
 	/*
 	 * Do not apply any pressure balancing cleverness when the
 	 * system is close to OOM, scan both anon and file equally
-- 
2.34.1