On Tue, Apr 12, 2022 at 1:06 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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?
