[RFC PATCH] mm/mglru: preserve inactive placement when enabling MGLRU

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

From: Ridong Chen <chenridong@xiaomi.com>

When the LRU is switched to MGLRU (echo y > /sys/kernel/mm/lru_gen/
enabled), fill_evictable() re-inserts every folio via
lru_gen_add_folio(..., false).  With reclaiming hardcoded to false, an
inactive anonymous folio (no PG_active, not in the swapcache) takes the
"gen = MIN_NR_GENS" branch in lru_gen_folio_seq() and is seeded at
seq = max_seq - 1, which lru_gen_is_active() treats as active.  Its
inactive placement is lost and NR_INACTIVE_ANON is folded into
NR_ACTIVE_ANON.

Pass reclaiming=!active so a folio from an inactive list is seeded into
an older generation.  Folios from the active list carry PG_active and
hit the first branch either way, so they are unchanged.

Tested on x86_64, next-20260812, 2G VM + 1G swap, ~1.5G anon pushed onto
the inactive list before enabling MGLRU:

                         Active(anon)  Inactive(anon)
  before switch (legacy)        2952         1548792   kB
  after `echo y`, unpatched  1552052               0   kB
  after `echo y`, patched      15144         1536636   kB

Inactive file folios stay inactive either way (NR_INACTIVE_FILE is
preserved).

Fixes: 354ed5974429 ("mm: multi-gen LRU: kill switch")
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Ridong Chen <chenridong@xiaomi.com>
---
 mm/vmscan.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 94fc4f25e99f..2befc8d7dd3f 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -5319,7 +5319,12 @@ static bool fill_evictable(struct lruvec *lruvec)
 			VM_WARN_ON_ONCE_FOLIO(folio_lru_gen(folio) != -1, folio);
 
 			lruvec_del_folio(lruvec, folio);
-			success = lru_gen_add_folio(lruvec, folio, false);
+			/*
+			 * Keep a folio from the inactive list inactive:
+			 * pass reclaiming=!active so it is not seeded as
+			 * active.  See lru_gen_folio_seq().
+			 */
+			success = lru_gen_add_folio(lruvec, folio, !active);
 			VM_WARN_ON_ONCE(!success);
 
 			if (!--remaining)
-- 
2.34.1