Re: [RFC PATCH] mm/mglru: preserve inactive placement when enabling MGLRU
From: Ridong Chen
Date: Thu Aug 13 2026 - 22:22:36 EST
On 8/13/2026 7:37 PM, Barry Song wrote:
On Thu, Aug 13, 2026 at 7:24 PM Barry Song <baohua@xxxxxxxxxx> wrote:
On Thu, Aug 13, 2026 at 7:02 PM Ridong Chen <ridong.chen@xxxxxxxxx> wrote:
From: Ridong Chen <chenridong@xxxxxxxxxx>
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@xxxxxxxxxx>
---
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);
Hi Barry, Thank you for your reply.
This is a very interesting use of the reclaim argument, as it is not
intended to serve this MGLRU switch purpose. It is really only meant
for `folio_rotate_reclaimable()`.
Yeah, I realize tying the reclaim argument to the MGLRU switch is a bit of a hack, since it was really only designed for folio_rotate_reclaimable(). That said, I'm not blind to it, I just posted an RFC patch to get the discussion rolling and see what people think.
Thanks for the suggestion. Traversing head to tail does fix the inversion, but we need to distinguish active from inactive during iteration, because they are inserted differently: list_add_tail() when reclaiming, and list_add() otherwise. That said, the reclaim argument usage still feels off to me—I'd like to hear if others have a better idea before we proceed with further changes.However, the change itself seems to be *partially* correct and
*partially* wrong.
One real issue is that inactive is always placed in the oldest
generation, while we have two old generations. Maybe we can ignore
this for now.
but somehow, are we also inverting the cold/hot ordering in the
inactive list?
`lru_to_folio(head)` always takes the tail, but now we are putting the
tail before the head folios.
Because reclaim == true will use list_add_tail().
if (reclaiming)
list_add_tail(&folio->lru, &lrugen->folios[gen][type][zone]);
I guess we can fix this by iterating in `fill_evictable()` from head
to tail order.
struct list_head *pos = head->next;
while (pos != head) {
struct folio *folio = list_entry(pos, struct folio, lru);
...
}
Then the oldest generation will maintain the same folio order as the
inactive list.
--
Best regards
Ridong