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

[Barry Song]

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);
>
> 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]);

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.

Thanks
Barry