Re: [PATCH v10 05/14] mm: multi-gen LRU: groundwork

[Yu Zhao <yuzhao@google.com>]

On Tue, Apr 12, 2022 at 1:06 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Mon, Apr 11, 2022 at 07:16:15PM -0700, Andrew Morton wrote:
>
> > > +{
> > > +   int gen;
> > > +   unsigned long old_flags, new_flags;
> > > +
> > > +   do {
> > > +           new_flags = old_flags = READ_ONCE(folio->flags);
> > > +           if (!(new_flags & LRU_GEN_MASK))
> > > +                   return false;
> > > +
> > > +           VM_BUG_ON_FOLIO(folio_test_active(folio), folio);
> > > +           VM_BUG_ON_FOLIO(folio_test_unevictable(folio), folio);
> > > +
> > > +           gen = ((new_flags & LRU_GEN_MASK) >> LRU_GEN_PGOFF) - 1;
> > > +
> > > +           new_flags &= ~LRU_GEN_MASK;
> > > +           /* for shrink_page_list() */
> > > +           if (reclaiming)
> > > +                   new_flags &= ~(BIT(PG_referenced) | BIT(PG_reclaim));
> > > +           else if (lru_gen_is_active(lruvec, gen))
> > > +                   new_flags |= BIT(PG_active);
> > > +   } while (cmpxchg(&folio->flags, old_flags, new_flags) != old_flags);
>
> Also; please use the form:
>
>         unsigned long new, old = READ_ONCE(folio->flags);
>
>         do {
>                 new = /* something */ old;
>         } while (!try_cmpxchg(&folio->flags, &old, new))

Sweet. Thanks.

A related question: if I pass new = old to try_cmpxchg(), does it know
that and avoid an unnecessary atomic op?