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

[Lokesh Gidra <lokeshgidra@google.com>]

On Tue, Sep 2, 2025 at 12:01 PM David Hildenbrand <david@redhat.com> wrote:
>
> On 02.09.25 20:59, Lokesh Gidra wrote:
> > 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 :)
>
> I'd suggest you prepare an RFC patch where you neatly summarize all we
> learned so far. :)
>
Sounds good. Will do. Thanks.
> --
> Cheers
>
> David / dhildenb
>