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

[Ridong Chen <ridong.chen@linux.dev>]

On 10/20/2026 4:29 PM, Barry Song wrote:
> On Tue, Oct 20, 2026 at 4:20 PM Kairui Song <ryncsn@gmail.com> wrote:
>>
>> On Tue, Oct 20, 2026 at 10:36 AM Ridong Chen <ridong.chen@linux.dev> wrote:
>>>
>>> From: Ridong Chen <chenridong@xiaomi.com>
>>>
>>> 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@xiaomi.com>
>>> ---
>>>   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.
>>

Fair points.

What confused me about the current table is that it is not clear
which dimension it is drawn along. The columns are generations, but
the rows mix access paths (page tables vs. file descriptors) with
the flags that protect each of them, and some of those, like
LRU_REFS_FLAGS, are not inputs to lru_gen_folio_seq() at all. So
when reading it next to the code, it is hard to tell which branch
puts a folio where.

How about keeping the existing table for the design overview, and
just adding a short note above it on what "age" means, without
hardcoding the number of generations, e.g.:

         /*
          * The folio is added to seq = max_seq + 1 - age, so age 1 is
          * the youngest generation. Ages up to MIN_NR_GENS are active,
          * and the result is clamped to min_seq[type].
          */

>>> +        *            +-----------------+-----------------+-----------------+-----------------+
>>> +        * 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.

Sounds good to me. Kairui, would you like to pick up the rename, and
the comment change above if you agree, as part of MGLRU-FG?

-- 
Best regards
Ridong