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

[Barry Song]

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);

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()`.

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]);


> VM_WARN_ON_ONCE(!success);
>
> if (!--remaining)
> --
> 2.34.1
>


Thanks
Barry