Re: [RFC PATCH v2 4/5] mm: mglru: run aging when pages are severely imbalanced across gens

[Kairui Song]

On Sun, Jul 26, 2026 at 8:21 PM Barry Song (Xiaomi) <baohua@xxxxxxxxxx> wrote:
>
> From: lyugaofei <lyugaofei@xxxxxxxxxx>
>
> This partially restores the reclaim behavior introduced in Yu
> Zhao's initial MGLRU commit, ac35a4902370 ("mm: multi-gen LRU:
> minimal implementation"):
>         /*
>          * It's also ideal to spread pages out evenly, i.e., 1/(MIN_NR_GENS+1)
>          * of the total number of pages for each generation. A reasonable range
>          * for this average portion is [1/MIN_NR_GENS, 1/(MIN_NR_GENS+2)]. The
>          * aging cares about the upper bound of hot pages, while the eviction
>          * cares about the lower bound of cold pages.
>          */
>         if (young * MIN_NR_GENS > total)
>                 return true;
>         if (old * (MIN_NR_GENS + 2) < total)
>                 return true;
>
> But with a stricter condition: the younger generations must contain
> at least four times as many folios as the older generations. This
> allows aging to keep folios of the preferred type spread across the
> reclaimable generations.
>
> Signed-off-by: lyugaofei <lyugaofei@xxxxxxxxxx>
> Co-developed-by: Barry Song (Xiaomi) <baohua@xxxxxxxxxx>
> Signed-off-by: Barry Song (Xiaomi) <baohua@xxxxxxxxxx>
> ---
>  mm/vmscan.c | 37 ++++++++++++++++++++++++++++++++++++-
>  1 file changed, 36 insertions(+), 1 deletion(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index d6bd64b4dced..7c13dedb0b1f 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -4956,9 +4956,40 @@ static int evict_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
>         return scanned;
>  }
>
> +static bool lru_gen_imbalanced(struct lruvec *lruvec, int type,
> +               unsigned long max_seq, unsigned long min_seq,
> +               int swappiness)
> +{
> +       struct lru_gen_folio *lrugen = &lruvec->lrugen;
> +       unsigned long young = 0, old = 0, seq;
> +
> +       /*
> +        * reclaim is forced to a single type in those cases, so there is
> +        * no need to consider the swappiness bias
> +        */
> +       if (swappiness == MIN_SWAPPINESS || swappiness > MAX_SWAPPINESS)
> +               return false;
> +
> +       for (seq = min_seq; seq <= max_seq; seq++) {
> +               int gen = lru_gen_from_seq(seq);
> +               unsigned long size = 0;
> +               int zone;
> +
> +               for (zone = 0; zone < MAX_NR_ZONES; zone++)
> +                       size += max(READ_ONCE(lrugen->nr_pages[gen][type][zone]), 0L);
> +
> +               if (seq + MIN_NR_GENS > max_seq)
> +                       young += size;
> +               else
> +                       old += size;
> +       }
> +       return young > old * 4;

Hi Barry,

MAX_NR_GEN could be tunable, would it be better to use a macro like
MAX_NR_GEN or MIN_NR_GEN here?

And I know there are experiments to extend MGLRU to run aging
periodically so each gen has a fixed age:

https://lwn.net/Articles/976985/

Not sure if this will conflict if this is a hardcoded aging behavior,
and if we want things like that in the future, these are useful on
servers, I think android might benefit foo.