Re: [PATCH] mm/mglru: restore accidentally removed seq < max_seq check

[Kairui Song]

On Tue, Sep 15, 2026 at 6:51 PM Barry Song (Xiaomi) <baohua@xxxxxxxxxx> wrote:
>
> Since commit 798c0330c2ca ("mm/mglru: rework aging feedback"),
> the following sanity check was accidentally removed:
>        if (seq < max_seq)
>                return 0;
>
> That means we can perform aging for any value less than or equal to
> max_gen_nr.
>
> This has been inconsistent with
> Documentation/admin-guide/mm/multigen_lru.rst, which states:
>
>  Users can write the following command to ``lru_gen`` to create a new
>  generation ``max_gen_nr+1``:
>
>      ``+ memcg_id node_id max_gen_nr [can_swap [force_scan]]``
>
> The correct semantics are that writing a value smaller than
> max_gen_nr should return 0, since the requested generation already
> exists.
>
> Fixes: 798c0330c2ca ("mm/mglru: rework aging feedback")
> Reported-by: Chuanhua Han <hanchuanhua802@xxxxxxxxx>
> Signed-off-by: Barry Song (Xiaomi) <baohua@xxxxxxxxxx>
> ---
>  mm/vmscan.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 5354eb8d3a07..18c4d3595749 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -5827,6 +5827,9 @@ static int run_aging(struct lruvec *lruvec, unsigned long seq,
>  {
>         DEFINE_MAX_SEQ(lruvec);
>
> +       if (seq < max_seq)
> +               return 0;
> +
>         if (seq > max_seq)
>                 return -EINVAL;

Reviewed-by: Kairui Song <kasong@xxxxxxxxxxx>