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

[Kiryl Shutsemau <kas@kernel.org>]

On Thu, Apr 23, 2026 at 02:57:34PM -0400, Peter Xu wrote:
> On Thu, Apr 23, 2026 at 07:08:00PM +0100, Kiryl Shutsemau wrote:
> > > - Whether read protection is required for an userspace swap system
> > >   (e.g. did you get time to have a look at umap?)
> > 
> > I looked at it briefly, so I can miss details.
> > 
> > IIUC, in absence of read tracking it doesn't collect hotness information
> > at all. The eviction is based on fault-in time: the oldest faulted-in
> 
> For example, let's imagine if we can have a per-mm idle page tracker, would
> it work for you to collect hotness info?
>
> The other idea is, no matter whether we use MGLRU or legacy LRU, if we can
> expose a better interface to share hotness info from kernel to userspace,
> would it be possible?

I don't see how either fits our problem.

Both page_idle and the LRUs (legacy or MGLRU) track accesses on physical
memory. We need visibility in the virtual address space domain.

We don't care which physical page backs a given guest address at any
moment. We want to know which piece of the user's dataset is cold, and
the answer has to be indifferent to kernel actions underneath: the
tracking must survive migration and swap-out. RWP gives us that — the
uffd-wp bit is preserved across swap PTEs and migration entries, so the
"this VA was declared cold" marker stays attached to the VA. A
physical-side tracker loses its state the moment the folio is freed or
replaced: a refaulted folio is a fresh object with no history.

Scaling goes the same way. Per-mm tracking of the form RWP does can
scale with the working set. A physical-side tracker scales with all folios
on the LRU/memcg, then needs an rmap walk per folio to map back to a
VA — which is exactly the reason page_idle doesn't scale for this use
case today.

There is also a cgroup-level confound: memcg hotness mixes guest memory
with the VMM's own (worker threads, I/O buffers, vhost-user rings).
VMA-scoped tracking is the natural unit regardless of the migration
story.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov