Re: [PATCH mm-new v2] mm: vmscan: put rotation-missed folios at the LRU tail

[Ridong Chen]

On 9/21/2026 5:31 AM, Barry Song wrote:
> On Sun, Sep 20, 2026 at 9:25 PM Ridong Chen <ridong.chen@xxxxxxxxx> wrote:
> >
> [...]
> >   mm/vmscan.c | 24 ++++++++++++++++++------
> >   1 file changed, 18 insertions(+), 6 deletions(-)
> >
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index e200ce3eb056..91295070ca33 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -1971,7 +1971,7 @@ static bool too_many_isolated(struct pglist_data *pgdat, int file,
> >    *
> >    * Note: The caller must not hold any lruvec lock.
> >    */
> > -static unsigned int move_folios_to_lru(struct list_head *list)
> > +static unsigned int move_folios_to_lru(struct list_head *list, bool do_rotate)
> >   {
> >          int nr_pages, nr_moved = 0;
> >          struct lruvec *lruvec = NULL;
> > @@ -2018,7 +2018,19 @@ static unsigned int move_folios_to_lru(struct list_head *list)
> >                          continue;
> >                  }
> >
> > -               lruvec_add_folio(lruvec, folio);
> > +               /*
> > +                * Put clean, unreferenced and unpinned folios that may have
> > +                * missed folio_rotate_reclaimable() at the tail to avoid
> > +                * cold/hot inversion.
> > +                */
> > +               if (do_rotate && !folio_test_active(folio) && !folio_mapped(folio) &&
> > +                   !folio_test_dirty(folio) && !folio_test_writeback(folio) &&
> > +                   !folio_test_referenced(folio) &&
> > +                   folio_ref_count(folio) == folio_expected_ref_count(folio))
>
>
> I guess this is wrong. We hold an extra reference while isolating
> the folio, so I think this should be:
>
> `folio_ref_count(folio) == folio_expected_ref_count(folio) + 1`
>
> Am I missing something here?
Hi Barry,

Thank you for your review.

The folios we want to check have the following lifecycle:

1. In isolate_lru_folios, we take an extra reference (i.e., +1).

2. In __remove_mapping, the folio can only be frozen successfully when refcount 
== 1 + folio_nr_pages(folio). We need to exclude folios whose refcount is not 1 
+ folio_nr_pages(folio) (e.g., those pinned by GUP), as reported by Sashiko.

```
	...
	refcount = 1 + folio_nr_pages(folio);
	if (!folio_ref_freeze(folio, refcount))
		goto cannot_free;
	...
``` 	

3. In move_folios_to_lru, we drop the extra reference and move the folio back to 
the lruvec.

```
...
		if (unlikely(folio_put_testzero(folio))) {
			__folio_clear_lru_flags(folio);
...
		}

		if (do_rotate && !folio_test_active(folio) && !folio_mapped(folio) &&
		    !folio_test_dirty(folio) && !folio_test_writeback(folio) &&
		    !folio_test_referenced(folio) &&
		    folio_ref_count(folio) == folio_expected_ref_count(folio))
			lruvec_add_folio_tail(lruvec, folio);
		else
			lruvec_add_folio(lruvec, folio);
```

The refcount check is added after the extra reference has been dropped.

Therefore, I believe it should be 'folio_ref_count(folio) == 
folio_expected_ref_count(folio)', not 'folio_ref_count(folio) == 
folio_expected_ref_count(folio) + 1'.

--
Best regards
Ridong