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

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

On 9/8/26 4:23 PM, Baoquan He wrote:
> On 09/08/26 at 03:53pm, Baolin Wang wrote:
>>
>>
>> On 9/8/26 2:59 PM, Baoquan He wrote:
>>> On 09/08/26 at 12:01pm, Baolin Wang wrote:
>>>>
>>>>
>>>> On 9/8/26 11:03 AM, Baolin Wang wrote:
>>>>>
>>>>>
>>>>> 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`?
>>>
>>> I would agree if we can. While I have one concern. A rejected folio that
>>> should have gone into the oldest generation is now being promoted to the
>>> 2nd newest generation. In the original code, referenced folio is only
>>> being promoted to the next gen. I even think this is not a bug, but Yu
>>
>> That's not quite true. Before commit 6cbdd9726fb5, a rejected referenced
>> folio was also put back to the 2nd youngest gen.
> 
> OK, I didn't follow your earlier discussion, I need take some time to
> fully understand that commit and the patch thread from Ehab.
> 
>>
>>> Zhao intentionally did it: the coldest folio is moved to 2nd newest gen,
>>> referenced folio (hot folio) is moved to new gen but carries the referenced
>>> bit. Both of them seems to be treated somewhat equally.
>>>
>>> To me, I would rather move both of them to the next gen, while keep their
>>> refs untouched.
>>
>> IMHO, I strongly recommend not doing this, and that's exactly the motivation
>> behind my patch. Because this is also being treated as a promotion, it
>> should behave like folio_inc_gen() or folio_update_gen() and clear the refs
>> after promotion. I think this is a fundamental principle of promotion.
>> Otherwise, ref-based promotion is already completely broken.
> 
> OK, it makes sense to me to make principle of promotion strictly applied
> no efficiency degradation involved.
> 
>>
>> Next, I also plan to clean up refs in lru_gen_set_refs() as discussed with
>> Barry.
> 
> Looks forward to seeing that. By the way, your discussion with Baryr is
> private or in public list, do you have pointer if public? Thanks.

I raised this issue before[1], and recently there has been another 
discussion[2] about it (we had some private discussions, but I'll post a 
new patch for discussion).

[1] 
https://lore.kernel.org/all/eb395442-0aad-428a-a5ac-9072d2d89060@linux.alibaba.com/
[2] 
https://lore.kernel.org/linux-mm/20260901220430.79810-1-baohua@kernel.org/

>>>>> 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?
>>>>
>>>> Just FYI, after above changes, the performance improvement on zram is no
>>>> longer obvious either. I think this also answers Kairui's earlier question
>>>> about why I saw a performance improvement (which seems related to commit
>>>> 6cbdd9726fb5). Also, there is no obvious performance regression either.
>>>
>>> Hi Baolin,
>>>
>>> Not sure if it's convenient to do a little more testing in your side.
>>> E.g rejected folios are moved to next gen, but not clearing their flags.
>>
>> I'm not sure what you mean by "next gen" here. If you mean the 2nd oldest
>> gen, I actually tested that too, that is, clearing refs after
>> move_folios_to_lru(), and there wasn't any noticeable performance impact
>> either (but the code was a bit hacky, so I didn't go with this approach).
> 
> Yeah, I meant the 2nd oldest gen, while what I am curious about is moving
> them into 2nd oldest gen but not clearing refs. Clearly it's conflicting
> with your plan. Anyway, it's just a brain store idea, please forget it.
> Thanks for the sharing and detailed explanation.

Thanks for reviewing.