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

David Hildenbrand (Arm)
2026-04-14T15:37:57Z

On 4/14/26 16:23, Kiryl Shutsemau (Meta) wrote:
> This series adds userfaultfd support for tracking the working set of
> VM guest memory, enabling VMMs to identify cold pages and evict them
> to tiered or remote storage.
>
> == Problem ==
>
> VMMs managing guest memory need to:
> 1. Track which pages are actively used (working set detection)
> 2. Safely evict cold pages to slower storage
> 3. Fetch pages back on demand when accessed again
>
> For shmem-backed guest memory, working set tracking partially works
> today: MADV_DONTNEED zaps PTEs while pages stay in page cache, and
> re-access auto-resolves from cache. But safe eviction still requires
> synchronous fault interception to prevent data loss races.
>
> For anonymous guest memory (needed for KSM cross-VM deduplication),
> there is no mechanism at all — clearing a PTE loses the page.
>
> == Solution ==
>
> The series introduces a unified userfaultfd interface that works
> across both anonymous and shmem-backed memory:
>
> UFFD_FEATURE_MINOR_ANON: extends MODE_MINOR registration to anonymous
> private memory. Uses the PROT_NONE hinting mechanism (same as NUMA
> balancing) to make pages inaccessible without freeing them.

I would rather tackle this from the other direction: it's another form
of protection (like WP), not really a "minor" mode.

Could we add a UFFDIO_REGISTER_MODE_RWP (or however we would call it)
and support it for anon+shmem, avoiding the zapping for shmem completely?

--
Cheers,

David