Re: [PATCH mm-new] mm/mglru: clarify the generation placement in lru_gen_folio_seq()

[Kairui Song]

On Tue, Oct 20, 2026 at 10:36 AM Ridong Chen <ridong.chen@xxxxxxxxx> wrote:
>
> From: Ridong Chen <chenridong@xxxxxxxxxx>
>
> lru_gen_folio_seq() computes how many generations below max_seq a
> folio should be added to, but stores it in a variable named "gen".
> Elsewhere in MGLRU "gen" means an index into lrugen->folios[], i.e.
> seq % MAX_NR_GENS, so reusing the name for a relative offset is
> confusing.
>
> The comment table above the code also no longer matches it. It only
> covers the PG_active and the final branch, leaving out the reclaiming
> and "can't evict yet" cases, and it lists LRU_REFS_FLAGS as an input
> although lru_gen_folio_seq() never looks at it.
>
> Rename the local variable to "age", redraw the table so that each
> branch maps to the age and seq it produces, and note that the result
> is clamped to min_seq[type].
>
> No functional change.
>
> Assisted-by: LLM
> Signed-off-by: Ridong Chen <chenridong@xxxxxxxxxx>
> ---
>  include/linux/mm_inline.h | 48 ++++++++++++++++++++++++++-------------
>  1 file changed, 32 insertions(+), 16 deletions(-)
>
> diff --git a/include/linux/mm_inline.h b/include/linux/mm_inline.h
> index ab69b9930893..a933d8c14558 100644
> --- a/include/linux/mm_inline.h
> +++ b/include/linux/mm_inline.h
> @@ -302,34 +302,50 @@ static inline unsigned long lru_gen_folio_seq(const struct lruvec *lruvec,
>                                               const struct folio *folio,
>                                               bool reclaiming)
>  {
> -       int gen;
> +       int age;
>         int type = folio_is_file_lru(folio);
>         const struct lru_gen_folio *lrugen = &lruvec->lrugen;
>
>         /*
> -        * +-----------------------------------+-----------------------------------+
> -        * | Accessed through page tables and  | Accessed through file descriptors |
> -        * | promoted by folio_update_gen()    | and protected by folio_inc_gen()  |
> -        * +-----------------------------------+-----------------------------------+
> -        * | PG_active (set while isolated)    |                                   |
> -        * +-----------------+-----------------+-----------------+-----------------+
> -        * |  PG_workingset  |  PG_referenced  |  PG_workingset  |  LRU_REFS_FLAGS |
> -        * +-----------------------------------+-----------------------------------+
> -        * |<---------- MIN_NR_GENS ---------->|                                   |
> -        * |<---------------------------- MAX_NR_GENS ---------------------------->|
> +        *            +-----------------+-----------------+-----------------+-----------------+
> +        * age        |        1        |        2        |        3        |        4        |
> +        *            +-----------------+-----------------+-----------------+-----------------+
> +        * seq        |     max_seq     |    max_seq-1    |    max_seq-2    |    max_seq-3    |

I think this comment update is not beneficial, we might have more than
4 gens in the furture, so hardcoding a number is not a good idea.

> +        *            +-----------------------------------+-----------------------------------+
> +        *            |<---------- MIN_NR_GENS ---------->|                                   |
> +        *            |<---------------------------- MAX_NR_GENS ---------------------------->|
> +        *            +-----------------------------------+-----------------------------------+
> +        *            | Accessed through page tables and  | Accessed through file descriptors |
> +        *            | promoted by folio_update_gen()    | and protected by folio_inc_gen()  |
> +        *            +-----------------+-----------------+-----------------+-----------------+
> +        * PG_active  |  PG_workingset  | !PG_workingset  |                 |                 |
> +        *            +-----------------+-----------------+-----------------+-----------------+
> +        * reclaiming |                 |                 |                 |     always      |
> +        *            +-----------------+-----------------+-----------------+-----------------+
> +        * can't      |                 |  anon && not in |                 |                 |
> +        * evict yet  |                 |  swapcache, or  |                 |                 |
> +        *            |                 |  PG_reclaim &&  |                 |                 |
> +        *            |                 | dirty/writeback |                 |                 |

I think it's better not to describe too many details in the comment as
it actually easier to just look at the code.

> +        *            +-----------------+-----------------+-----------------+-----------------+
> +        * else       |                 |                 |  PG_workingset  | !PG_workingset  |
> +        *            |                 |                 |       or        |       &&        |
> +        *            |                 |                 |  PG_referenced  | !PG_referenced  |
> +        *            +-----------------+-----------------+-----------------+-----------------+
> +        *
> +        * The resulting seq is clamped to min_seq[type].
>          */
>         if (folio_test_active(folio))
> -               gen = MIN_NR_GENS - folio_test_workingset(folio);
> +               age = MIN_NR_GENS - folio_test_workingset(folio);

That's a very nice rename, the `gen` variable is so confusing to me
for a long time, I thought about renaming it to `gap`, `age` looks
much better. Thanks!