Re: [PATCH] mm: migrate LRU_REFS_MASK bits in folio_migrate_flags

[Zhaoyang Huang]

On Wed, Sep 25, 2024 at 5:42 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Wed, 25 Sep 2024 11:02:25 +0800 "zhaoyang.huang" <zhaoyang.huang@xxxxxxxxxx> wrote:
>
> > From: Zhaoyang Huang <zhaoyang.huang@xxxxxxxxxx>
> >
> > Bits of LRU_REFS_MASK are not inherited during migration which lead to
> > new_folio start from tier0. Fix this by migrate the bits domain.
>
> I'm having trouble understanding this, sorry.  Please more fully
> describe the runtime effects of this flaw.
Sorry for bringing confusion. According to my understanding, MGLRU
records how many times that vfs access this page in a range of bits
domain(LRU_REFS_MASK) in folio->flags which are not being migrated to
new_folios so far. This commit would like to do so to have the
new_folio inherit these bits from the old folio. Is it right and
worthy to do?
>
> > --- a/include/linux/mm_inline.h
> > +++ b/include/linux/mm_inline.h
> > @@ -291,6 +291,12 @@ static inline bool lru_gen_del_folio(struct lruvec *lruvec, struct folio *folio,
> >       return true;
> >  }
> >
> > +static inline void folio_migrate_refs(struct folio *new_folio, struct folio *folio)
> > +{
> > +     unsigned long refs = READ_ONCE(folio->flags) & LRU_REFS_MASK;
> > +
> > +     set_mask_bits(&new_folio->flags, LRU_REFS_MASK, refs);
> > +}
> >  #else /* !CONFIG_LRU_GEN */
> >
> >  static inline bool lru_gen_enabled(void)
> > @@ -313,6 +319,8 @@ static inline bool lru_gen_del_folio(struct lruvec *lruvec, struct folio *folio,
> >       return false;
> >  }
> >
> > +static inline void folio_migrate_refs(struct folio *new_folio, struct folio *folio)
> > +{}
> >  #endif /* CONFIG_LRU_GEN */
> >
> >  static __always_inline
> > diff --git a/mm/migrate.c b/mm/migrate.c
> > index 923ea80ba744..60c97e235ae7 100644
> > --- a/mm/migrate.c
> > +++ b/mm/migrate.c
> > @@ -618,6 +618,7 @@ void folio_migrate_flags(struct folio *newfolio, struct folio *folio)
> >       if (folio_test_idle(folio))
> >               folio_set_idle(newfolio);
> >
> > +     folio_migrate_refs(newfolio, folio);
> >       /*
> >        * Copy NUMA information to the new page, to prevent over-eager
> >        * future migrations of this same page.
> > --
> > 2.25.1