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

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

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`?
> 
> 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.

zram swap:
		w/o patch	w/ patch
sys time:	1666.5s		1628.5s