On Tue, Apr 14, 2026 at 06:10:44PM +0100, Kiryl Shutsemau wrote: > On Tue, Apr 14, 2026 at 05:37:50PM +0200, David Hildenbrand (Arm) wrote: > > 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? > > I like this idea. > > It should be functionally equivalent, but your interface idea fits > better with the rest. > > Thanks! Will give it a try.
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. -- Kiryl Shutsemau / Kirill A. Shutemov
