Re: [PATCH v3 6/7] mm/mglru: move folios from oldest gen to second-oldest gen from head to tail

[Barry Song]

On Fri, Sep 4, 2026 at 9:53 AM Baolin Wang
<baolin.wang@xxxxxxxxxxxxxxxxx> wrote:
>
>
>
> On 9/2/26 7:24 AM, Barry Song (Xiaomi) wrote:
> > For reclamation, it makes sense to reclaim folios from tail to
> > head, as folios near the head are relatively hot. However, when
> > moving folios from the oldest generation to the second-oldest
> > generation, using the tail-to-head order would effectively cause
> > a cold/hot inversion.
> >
> > Signed-off-by: Barry Song (Xiaomi) <baohua@xxxxxxxxxx>
> > Reviewed-by: Baoquan He <baoquan.he@xxxxxxxxx>
> > Tested-by: Xueyuan Chen <xueyuan.chen21@xxxxxxxxx>
> > Reviewed-by: Lian Wang <lianux.mm@xxxxxxxxx>
> > ---
> >   mm/vmscan.c | 23 +++++++++++++++++++++--
> >   1 file changed, 21 insertions(+), 2 deletions(-)
> >
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 1907a946840d..76dfa9575852 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -192,11 +192,27 @@ static inline void prefetchw_prev_lru_folio(struct folio *folio,
> >               prefetchw(&prev->flags);
> >       }
> >   }
> > +
> > +static inline void prefetchw_next_lru_folio(struct folio *folio,
> > +             struct list_head *base)
> > +{
> > +     if (folio->lru.next != base) {
> > +             struct folio *next;
> > +
> > +             next = list_entry(folio->lru.next, struct folio, lru);
> > +             prefetchw(&next->flags);
> > +     }
> > +}
>
> You did not mention in the commit message why this prefetch was added.
> Just curious, does it really help performance?

Hi Baolin,

Thanks for your review.

I had this discussion with Kairui and thought it might be
useful to keep it here:

https://lore.kernel.org/linux-mm/CAMgjq7AJnWGGH=FNuV7ynXjbffktsDRgdcyagLUJUwJf1ukaJQ@xxxxxxxxxxxxxx/

It might be arch-dependent. Some architectures could benefit
significantly from prefetching, while others might see little to
no impact.

for my x86 test, it has very slight improvement:

***** no-prefetch:

agetest:
...
gen 100:    2.421 ms
gen 101:    2.424 ms
gen 102:    2.420 ms
gen 103:    2.413 ms

Total:   248.418 ms
Average: 2.460 ms

agetest:
...
gen 100:    2.393 ms
gen 101:    2.396 ms
gen 102:    2.395 ms
gen 103:    2.392 ms

Total:   245.627 ms
Average: 2.432 ms

agetest:
...
gen 100:    2.433 ms
gen 101:    2.427 ms
gen 102:    2.432 ms
gen 103:    2.450 ms

Total:   249.186 ms
Average: 2.467 ms

**** has-prefetch:

agetest:
....
gen 100:    2.314 ms
gen 101:    2.310 ms
gen 102:    2.321 ms
gen 103:    2.303 ms

Total:   237.619 ms
Average: 2.353 ms

agetest:
gen 100:    2.342 ms
gen 101:    2.343 ms
gen 102:    2.339 ms
gen 103:    2.335 ms

Total:   239.929 ms
Average: 2.376 ms

agetest:

gen 100:    2.344 ms
gen 101:    2.347 ms
gen 102:    2.352 ms
gen 103:    2.348 ms

Total:   241.188 ms
Average: 2.388 ms

Basically, it’s 2.3xx vs. 2.4xx, lower is better.

Best Regards
Barry