Re: [RFC, PATCH 00/12] userfaultfd: working set tracking for VM guest memory

Kiryl Shutsemau
2026-04-17T12:26:39Z

On Fri, Apr 17, 2026 at 01:43:36PM +0200, David Hildenbrand (Arm) wrote:
> On 4/16/26 22:25, Kiryl Shutsemau wrote:
> > On Thu, Apr 16, 2026 at 08:32:19PM +0200, David Hildenbrand (Arm) wrote:
> >> On 4/16/26 15:49, Kiryl Shutsemau wrote:
> >>>
> >>> Here is an updated version:
> >>>
> >>> https://git.kernel.org/pub/scm/linux/kernel/git/kas/linux.git/log/?h=uffd/rfc-v2
> >>>
> >>> will post after -rc1 is tagged.
> >>>
> >>> I like it more. It got substantially cleaner.
> >>
> >> I don't have time to look into the details just yet, but my thinking was
> >> that
> >>
> >> a) It would avoid the zap+refault
> >
> > Yep.
> >
> >> b) We could reuse the uffd-wp PTE bit + marker to indicate/remember the
> >> protection, making it co-exist with NUMA hinting naturally.
> >>
> >> b) obviously means that we cannot use uffd-wp and uffd-rwp at the same
> >> time in the same uffd area. I guess that should be acceptable for the
> >> use cases we you should have in mind?
> >
> > I took a different path: I still use PROT_NONE PTEs, so it cannot
> > co-exist with NUMA balancing [fully], but WP + RWP should be fine. I
> > need to add a test for this.
> >
> > I didn't give up on NUMA balancing completely. task_numa_fault() is
> > called on RWP fault. So it should help scheduler decisions somewhat.
> >
> > I think an RWP user might want to use WP too.
> >
> > Do you see this trade-off as reasonable?
>
> One reason why the PTE bit was added for the WP case was to distinguish
> it from other write faults.
>
> I assume without a dedicated PTE bit your design will always suffer from
> false positive notifications.
>
> Leaving NUMA-balancing aside, a simple
> mprotect(PROT_NONE)+mprotect(PROT_READ) would already be problematic to
> distinguish both cases.

Hm. I didn't consider this case (miss some uffd lore). Will rework to
reuse existing PTE bit.

Thanks for the feedback!

--
Kiryl Shutsemau / Kirill A. Shutemov