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

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

On 10/21/2026 4:38 PM, wangzicheng@honor.com wrote:
> On 2026-10-20 10:35 +0800, Ridong Chen 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        |
> Hi Ridong,
>    
> Agreed, I also found this part confusing.
> 
> One problem with the new name: in MGLRU a larger seq/gen means a
> younger generation, but a larger age means an older one, so every
> read needs a flip (max_seq - age + 1). Assigning the seq directly in
> each branch might read more naturally.
> 

Hi Zicheng,

Thank you for your reply. We both agree that this part of the code is confusing. 
'age' is not perfect, but it is much better. The bigger the age, the elder it 
is, just like human age. And it is a local variable, not used elsewhere, so it 
is fine for me.

> Also, both tables run past 80 columns (~95 for the art in mglru-fg
> 4/17, ~96 for the redrawn one here, tab as 8). Would you consider
> something vertical and shorter? I had prepared a comment for
> mglru-fg, and since this is being discussed here too, here it is
> (conditions are per mglru-fg, where the flags became the refs
> count):
> 
>      /*
>          * The seq is clamped to min_seq[type]:
>          *
>          *   destination     | folio state
>          *   ----------------+--------------------------------------------
>          *   youngest        | PG_active, refs >= LRU_REFS_WORKINGSET
>          *   (max_seq)       |
>          *
>          *   2nd youngest    | PG_active, refs < LRU_REFS_WORKINGSET
>          *   (max_seq - 1)   | anon folio not yet in swapcache
>          *                   | PG_reclaim folio, dirty or under writeback
>          *
>          *   2nd oldest      | refs >= LRU_REFS_WORKINGSET
>          *   (max_seq - 2)   |
>          *
>          *   oldest          | reclaiming
>          *   (min_seq[type]) | otherwise: refs < LRU_REFS_WORKINGSET
>          */
> 
> With a comment like this, developers can see the generation
> placement directly, instead of working it out from the code.
> 

Thanks for the table,

As Kairui replied, he did not want more code details in the comment, which may 
change a lot. And Kairui will pick this patch with his FG series. Although I 
wanted to add the table, Kairui felt there shouldn't be too much detail.

What is more, I think the following is needed—it is what this function initially 
implemented.

+-----------------------------------+-----------------------------------+
| Accessed through page tables and  | Accessed through file descriptors |
| promoted by folio_update_gen()    | and protected by folio_inc_gen()  |
+-----------------+-----------------+-----------------+-----------------+

> Best,
> Zicheng
> 
> 
>> +	 *            +-----------------+-----------------+-----------------+-----------------+
>> +	 * seq        |     max_seq     |    max_seq-1    |    max_seq-2    |    max_seq-3    |
>> +	 *            +-----------------------------------+-----------------------------------+
>> +	 *            |<---------- 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 |                 |                 |
>> +	 *            +-----------------+-----------------+-----------------+-----------------+
>> +	 * 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);
>>   	else if (reclaiming)
>> -		gen = MAX_NR_GENS;
>> +		age = MAX_NR_GENS;
>>   	else if ((!folio_is_file_lru(folio) && !folio_test_swapcache(folio)) ||
>>   		 (folio_test_reclaim(folio) &&
>>   		  (folio_test_dirty(folio) || folio_test_writeback(folio))))
>> -		gen = MIN_NR_GENS;
>> +		age = MIN_NR_GENS;
>>   	else
>> -		gen = MAX_NR_GENS - (folio_test_workingset(folio) || folio_test_referenced(folio));
>> +		age = MAX_NR_GENS - (folio_test_workingset(folio) || folio_test_referenced(folio));
>>   
>> -	return max(READ_ONCE(lrugen->max_seq) - gen + 1, READ_ONCE(lrugen->min_seq[type]));
>> +	return max(READ_ONCE(lrugen->max_seq) - age + 1, READ_ONCE(lrugen->min_seq[type]));
>>   }
>>   
>>   static inline bool lru_gen_add_folio(struct lruvec *lruvec, struct folio *folio, bool reclaiming)
>> -- 
>> 2.34.1
>>
>>
> 

-- 
Best regards
Ridong