Re: [PATCH v6 05/11] mm: Add fast_only bool to test_young and clear_young MMU notifiers

[James Houghton <jthoughton@google.com>]

On Fri, Aug 2, 2024 at 8:57 AM David Hildenbrand <david@redhat.com> wrote:
>
> On 02.08.24 01:13, James Houghton wrote:
> > On Thu, Aug 1, 2024 at 2:36 AM David Hildenbrand <david@redhat.com> wrote:
> >>
> >> On 24.07.24 03:10, James Houghton wrote:
> >>> For implementers, the fast_only bool indicates that the age information
> >>> needs to be harvested such that we do not slow down other MMU operations,
> >>> and ideally that we are not ourselves slowed down by other MMU
> >>> operations.  Usually this means that the implementation should be
> >>> lockless.
> >>
> >> But what are the semantics if "fast_only" cannot be achieved by the
> >> implementer?
> >>
> >> Can we add some documentation to the new functions that explain what
> >> this mysterious "fast_only" is and what the expected semantics are?
> >> Please? :)
> >
> > Thanks for pointing out the missing documentation. How's this?
> >
> > diff --git a/include/linux/mmu_notifier.h b/include/linux/mmu_notifier.h
> > index 45c5995ebd84..c21992036dd3 100644
> > --- a/include/linux/mmu_notifier.h
> > +++ b/include/linux/mmu_notifier.h
> > @@ -106,6 +106,18 @@ struct mmu_notifier_ops {
> >           * clear_young is a lightweight version of clear_flush_young. Like the
> >           * latter, it is supposed to test-and-clear the young/accessed bitflag
> >           * in the secondary pte, but it may omit flushing the secondary tlb.
> > +        *
>
> Probably makes sense to highlight the parameters like @fast_only

Will do.

>
> > +        * The fast_only parameter indicates that this call should not block,
> > +        * and this function should not cause other MMU notifier calls to
> > +        * block. Usually this means that the implementation should be
> > +        * lockless.
> > +        *
> > +        * When called with fast_only, this notifier will be a no-op unless
> > +        * has_fast_aging is set on the struct mmu_notifier.
>
> "... and will return 0 (NOT young)." ?

Thanks, I'll add this.

>
> > +        *
> > +        * When fast_only is true, if the implementer cannot determine that a
> > +        * range is young without blocking, it should return 0 (i.e.,
> > +        * that the range is NOT young).
> >           */
> >          int (*clear_young)(struct mmu_notifier *subscription,
> >                             struct mm_struct *mm,
> > @@ -118,6 +130,8 @@ struct mmu_notifier_ops {
> >           * the secondary pte. This is used to know if the page is
> >           * frequently used without actually clearing the flag or tearing
> >           * down the secondary mapping on the page.
> > +        *
> > +        * The fast_only parameter has the same meaning as with clear_young.
> >           */
> >          int (*test_young)(struct mmu_notifier *subscription,
> >                            struct mm_struct *mm,
> >
> > I've also moved the commit that follows this one (the one that adds
> > has_fast_aging) to be before this one so that the comment makes sense.
>
>
> Makes sense, thanks!

Thanks David!