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

[Barry Song]

On Tue, Oct 20, 2026 at 4:20 PM Kairui Song <ryncsn@xxxxxxxxx> wrote:
>
> 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!

Agreed with the rename part. Maybe this doesn't conflict much
with MGLRU-FG either. It could be cherry-picked before
MGLRU-FG or included as part of the MGLRU-FG series.
I'd vote for the latter.