Re: [RFC PATCH] mm: allow smaller large folios to use lru_cache

[Barry Song]

On Thu, Jul 2, 2026 at 9:00 PM David Hildenbrand (Arm) <david@xxxxxxxxxx> wrote:
[...]
> > diff --git a/include/linux/swap.h b/include/linux/swap.h
> > index 8f0f68e245ba..fce1d84aa2c8 100644
> > --- a/include/linux/swap.h
> > +++ b/include/linux/swap.h
> > @@ -322,9 +322,9 @@ static inline bool folio_may_be_lru_cached(struct folio *folio)
> >       /*
> >        * Holding PMD-sized folios in per-CPU LRU cache unbalances accounting.
> >        * Holding small numbers of low-order mTHP folios in per-CPU LRU cache
> > -      * will be sensible, but nobody has implemented and tested that yet.
> > +      * will be sensible.
> >        */
> > -     return !folio_test_large(folio);
> > +     return folio_order(folio) < PAGE_ALLOC_COSTLY_ORDER;
> >  }
> >
> >  extern atomic_t lru_disable_count;
>
> Not that __wp_can_reuse_large_anon_folio() currently assumes that large folios
> are not in the LRU cache.

Thanks for pointing this out. I assume we could do something
similar to wp_can_reuse_anon_folio(). It won't always help
drain lru_cache, but it could help in some cases?

diff --git a/mm/memory.c b/mm/memory.c
index a29cd38ad547..db82ac8057ac 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4143,6 +4143,12 @@ static bool
__wp_can_reuse_large_anon_folio(struct folio *folio,
                folio_unlock(folio);
        }

+       if (folio_may_be_lru_cached(folio) && !folio_test_lru(folio)) {
+               if (folio_ref_count(folio) > folio_large_mapcount(folio) + 1)
+                       return false;
+               lru_add_drain();
+       }
+
        if (folio_large_mapcount(folio) != folio_ref_count(folio))
                return false;

Best Regards
Barry