Re: [PATCH] mm: mglru: clear the reference counter for rejected folios

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 9/8/26 10:34 AM, Barry Song wrote:
> On Tue, Sep 8, 2026 at 10:30 AM Baoquan He <baoquan.he@linux.dev> wrote:
>>
>> Hi Baolin,
>>
>> On 09/07/26 at 11:25am, Baolin Wang wrote:
>> ......snip...
>>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>>> index 40d3f1b48a74..42c0a09938ab 100644
>>> --- a/mm/vmscan.c
>>> +++ b/mm/vmscan.c
>>
>> Well, this seems to be based on Andrew's mm-new branch. I usually track
>> mm-unstable branch. Maybe the subject should be marked as below?
>> [PATCH mm-new] mm: mglru: clear the reference counter for rejected

ACK.

>>
>>> @@ -5021,10 +5021,11 @@ static int evict_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
>>>                }
>>>
>>>                /* don't add rejected folios to the oldest generation */
>>> -             if (lru_gen_folio_seq(lruvec, folio, false) == min_seq[type]) {
>>> -                     folio_set_lru_refs(folio, 0);
>>> +             if (lru_gen_folio_seq(lruvec, folio, false) == min_seq[type])
>>>                        folio_set_active(folio);
>>> -             }
>>> +
>>> +             /* See the comments on LRU_REFS_FLAGS */
>>> +             folio_set_lru_refs(folio, 0);
>>
>> This looks like a great catch, while the code change could bring issue.
>>
>> Because move_folios_to_lru() relies on folios' flags to decide their new
>> generation. You just cleared it before move_folios_to_lru(). This is no
>> problem for rejected folios that are determined to be put into the
>> oldest generation. But for those rejected folios that are determined to
>> be promoted, this could be wrong. E.g currently gen window is 4, and a
>> folio is referenced, lru_gen_folio_seq() decides its new gen as 1, which
>> is the 2nd oldest generation. While folio_set_lru_refs(folio, 0) clear
>> referenced bit, this causes it being put into the oldest generation in
>> move_folios_to_lru(), this is not expected.

Yes. As I discussed with Barry earlier, lru_gen_folio_seq() also needs 
to be reconsidered regarding whether it should rely on PG_referenced [1].

For commit 6cbdd9726fb5, we didn't discuss the impact on rejected folios 
either. Before commit 6cbdd9726fb5, if rejected folios did not have 
PG_active set by shrink_folio_list(), evict_folios() would set PG_active 
on these rejected folios.

[1] 
https://lore.kernel.org/linux-mm/20260901220430.79810-1-baohua@kernel.org/

> The original code looks quite weird. It even prioritizes folios
> that won't be promoted by `PG_active`. Do we need to change all
> the cases just to call `PG_active`?

Yes. Regarding this concern, I plan to change back to the original behavior:

	/* See the comments on LRU_REFS_FLAGS */
	folio_set_lru_refs(folio, 0);

	/* don't add rejected folios to the oldest generation */
	if (lru_gen_folio_seq(lruvec, folio, false) == min_seq[type])
		folio_set_active(folio);

What do you think?