Re: [DISCUSSION] Unconditionally lock folios when calling rmap_walk()

[Lokesh Gidra <lokeshgidra@google.com>]

On Fri, Aug 29, 2025 at 2:04 AM Lorenzo Stoakes
<lorenzo.stoakes@oracle.com> wrote:
>
> On Fri, Aug 29, 2025 at 10:42:45AM +0200, David Hildenbrand wrote:
> > > >
> > > > I do wonder if we can identify this case and handle things differently.
> > > >
> > > > Perhaps even saying 'try and get the rmap lock, but if there's "too much"
> > > > contention, grab the folio lock.
> > >
> > > Can you please elaborate what you mean? Where do you mean we can
> > > possibly do something like this?
> > >
> > > UFFD move only works on PageAnonExclusive folios. So, would it help
> > > (in terms of avoiding contention) if we were to change the condition:
> >
> > I think we shouldn't be using PAE here. Once could consider using
> > folio_maybe_mapped_shared(), and assume contention on the folio lock if it
> > is maybe mapped shared.
>
> Interesting!
>
> >
> > But the real question is with whom we would be contending for the folio
> > lock.
> >
> > Is it really other processes mapping that folio? I'm not so sure.
>
> Yeah, I might go off and do some research myself on this, actually. Nail down
> wehre this might actually happen.
>
> Generally I'm softening on this and maybe we're good with the proposed change.
>
> But still want to be super careful here... :)
>
Anxiously waiting for your assessment. Fingers crossed :)
> >
> > --
> > Cheers
> >
> > David / dhildenb
> >
>
> Cheers, Lorenzo