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

Kiryl Shutsemau
2026-04-27T10:23:14Z

On Sat, Apr 25, 2026 at 08:05:16AM +0200, David Hildenbrand (Arm) wrote:
> On 4/24/26 15:49, Kiryl Shutsemau wrote:
> > On Fri, Apr 24, 2026 at 07:51:44AM -0400, Peter Xu wrote:
> >> On Fri, Apr 24, 2026 at 11:34:48AM +0100, Kiryl Shutsemau wrote:
> >>> Both page_idle and the LRUs (legacy or MGLRU) track accesses on physical
> >>> memory. We need visibility in the virtual address space domain.
> >>
> >> Yes they are, but ACCESS bit isn't.
> >
> > A-bit is not a reliable signal for userspace working-set tracking
> > because the kernel itself is a concurrent consumer.
> Right, I don't think we want to rely on either the A bit just like we don't want
> to rely on the Dirty bit in other code. (and even SoftDirty bit is a flawed concept)
>
> I do see some value in a reliable RWP mechanism based on uffd. The real question
> is, how much benefit it would bring (which other use cases could benefit from it).

I have not put much thought into use-cases beyond VMs, but I think other
mmap-heavy loads (databases, GC'd runtimes, etc) can make a use in a
similar way: workset tracking and organizing userspace-driven memory
tiering based on the workset data.

--
Kiryl Shutsemau / Kirill A. Shutemov