[PATCH 00/14] userfaultfd: working set tracking for VM guest memory

Kiryl Shutsemau (Meta)
2026-04-27T11:46:20Z

This series adds userfaultfd support for tracking the working set of
VM guest memory, so a VMM can identify cold pages and evict them to
tiered or remote storage.

== Problem ==

A VMM managing guest memory needs to:

1. detect which pages are still being touched (working-set
tracking);
2. safely evict cold pages to slower tiered or remote storage;
3. fetch them back on demand when accessed again.

== Approach ==

UFFDIO_REGISTER_MODE_RWP is a new userfaultfd registration mode, in
parallel with the existing MODE_MISSING / MODE_WP / MODE_MINOR. It
uses the same mechanism on every backing -- anon, shmem, hugetlbfs:

- PAGE_NONE on the PTE (the same primitive NUMA balancing uses)
makes the page inaccessible while keeping it resident;
- the uffd PTE bit (the one MODE_WP already owns) marks the entry
as "userfaultfd-tracked" so the protnone fault path can tell an
RWP fault apart from an mprotect(PROT_NONE) or NUMA hinting
fault.

VM_UFFD_WP and VM_UFFD_RWP are mutually exclusive per VMA, so the
same PTE bit safely carries both meanings depending on the
registered VMA flag.

In sync mode, the kernel delivers a UFFD_PAGEFAULT_FLAG_RWP message
to the registered handler, and the handler resolves the fault with
UFFDIO_RWPROTECT clearing MODE_RWP. In async mode
(UFFD_FEATURE_RWP_ASYNC), the fault is auto-resolved in-place: the
kernel restores the original PTE permissions and the faulting thread
continues without a userfaultfd message ever being delivered.
Userspace then learns which pages were touched by reading
PAGE_IS_ACCESSED out of PAGEMAP_SCAN -- pages whose uffd bit is
still set were not re-accessed since the last RWP cycle.

UFFDIO_RWPROTECT is the protect/unprotect ioctl, mirroring
UFFDIO_WRITEPROTECT.

UFFDIO_SET_MODE flips RWP_ASYNC <-> sync at runtime under
mmap_write_lock(), so a VMM can run in async mode for detection and switch
to sync for race-free eviction without re-registering the userfaultfd.

== Typical VMM workflow ==

/* arm */
UFFDIO_API(features = RWP | RWP_ASYNC)
UFFDIO_REGISTER(MODE_RWP)

/* detection cycle */
UFFDIO_RWPROTECT(range, RWP)
sleep(interval)
PAGEMAP_SCAN(!PAGE_IS_ACCESSED) -> cold pages

/* eviction */
UFFDIO_SET_MODE(disable = RWP_ASYNC) /* sync */
pwrite(cold) + fallocate(FALLOC_FL_PUNCH_HOLE, cold) /* races trapped */
UFFDIO_SET_MODE(enable = RWP_ASYNC) /* resume */

== Series layout ==

Patches 1 to 3 are preparatory:

1: decouple protnone helpers from CONFIG_NUMA_BALANCING.
2-3: rename _PAGE_BIT_UFFD_WP, pte_uffd_wp() and friends to drop the
_WP suffix, since the bit now carries WP and RWP meaning
depending on the VMA flag. The SCAN_PTE_UFFD enum's ftrace
output string is intentionally kept as "pte_uffd_wp" so
trace-based tooling does not silently break.

Patches 4 to 7 add the in-kernel mechanism:

4: VM_UFFD_RWP VMA flag and CONFIG_USERFAULTFD_RWP.
5: MM_CP_UFFD_RWP change_protection() primitive (PAGE_NONE +
uffd bit, plus a RESOLVE counterpart).
6: marker preservation across swap, device-exclusive, migration,
fork, mremap, UFFDIO_MOVE, hugetlb copy, and mprotect().
7: handle VM_UFFD_RWP in khugepaged, rmap, and GUP.

Patches 8 to 12 wire the userspace surface:

8: UFFDIO_REGISTER_MODE_RWP and UFFDIO_RWPROTECT plumbing.
9: RWP fault delivery and exposure of UFFDIO_REGISTER_MODE_RWP.
10: PAGE_IS_ACCESSED in PAGEMAP_SCAN.
11: UFFD_FEATURE_RWP_ASYNC for async fault resolution.
12: UFFDIO_SET_MODE for runtime sync/async toggle.

Patches 13 and 14 are tests and documentation.

== Notable changes since the RFC ==

- MODE_MINOR-extension dropped in favour of a dedicated MODE_RWP;
UFFDIO_DEACTIVATE replaced by UFFDIO_RWPROTECT (mirror of
UFFDIO_WRITEPROTECT).
- shmem and hugetlbfs are no longer zapped on protect; they go
through the same PAGE_NONE + uffd-bit mechanism as anon, so the
RWP semantics are uniform across all three backings.
- PAGE_IS_UFFD_DEACTIVATED in PAGEMAP_SCAN renamed to
PAGE_IS_ACCESSED; the value reported is now an "accessed since
the last RWP cycle" signal, scoped to VM_UFFD_RWP VMAs and
distinct from PAGE_IS_WRITTEN.
- SET_MODE scope reduced to UFFD_FEATURE_RWP_ASYNC only; the
runtime escalation path that auto-enabled WP_UNPOPULATED has
been removed.

Kiryl Shutsemau (Meta) (14):
mm: decouple protnone helpers from CONFIG_NUMA_BALANCING
mm: rename uffd-wp PTE bit macros to uffd
mm: rename uffd-wp PTE accessors to uffd
mm: add VM_UFFD_RWP VMA flag
mm: add MM_CP_UFFD_RWP change_protection() flag
mm: preserve RWP marker across PTE rewrites
mm: handle VM_UFFD_RWP in khugepaged, rmap, and GUP
userfaultfd: add UFFDIO_REGISTER_MODE_RWP and UFFDIO_RWPROTECT
plumbing
mm/userfaultfd: add RWP fault delivery and expose
UFFDIO_REGISTER_MODE_RWP
mm/pagemap: add PAGE_IS_ACCESSED for RWP tracking
userfaultfd: add UFFD_FEATURE_RWP_ASYNC for async fault resolution
userfaultfd: add UFFDIO_SET_MODE for runtime sync/async toggle
selftests/mm: add userfaultfd RWP tests
Documentation/userfaultfd: document RWP working set tracking

Documentation/admin-guide/mm/pagemap.rst | 13 +-
Documentation/admin-guide/mm/userfaultfd.rst | 211 +++++-
Documentation/filesystems/proc.rst | 1 +
arch/arm64/Kconfig | 1 +
arch/arm64/include/asm/pgtable-prot.h | 8 +-
arch/arm64/include/asm/pgtable.h | 47 +-
arch/loongarch/Kconfig | 1 +
arch/loongarch/include/asm/pgtable.h | 4 +-
arch/powerpc/include/asm/book3s/64/pgtable.h | 8 +-
arch/powerpc/platforms/Kconfig.cputype | 1 +
arch/riscv/Kconfig | 1 +
arch/riscv/include/asm/pgtable-bits.h | 12 +-
arch/riscv/include/asm/pgtable.h | 59 +-
arch/s390/Kconfig | 1 +
arch/s390/include/asm/hugetlb.h | 12 +-
arch/s390/include/asm/pgtable.h | 4 +-
arch/x86/Kconfig | 1 +
arch/x86/include/asm/pgtable.h | 56 +-
arch/x86/include/asm/pgtable_types.h | 16 +-
fs/proc/task_mmu.c | 98 ++-
fs/userfaultfd.c | 246 ++++++-
include/asm-generic/hugetlb.h | 18 +-
include/asm-generic/pgtable_uffd.h | 32 +-
include/linux/huge_mm.h | 7 +
include/linux/leafops.h | 4 +-
include/linux/mm.h | 46 +-
include/linux/mm_inline.h | 4 +-
include/linux/pgtable.h | 22 +-
include/linux/swapops.h | 4 +-
include/linux/userfaultfd_k.h | 76 +-
include/trace/events/huge_memory.h | 2 +-
include/trace/events/mmflags.h | 7 +
include/uapi/linux/fs.h | 1 +
include/uapi/linux/userfaultfd.h | 54 +-
init/Kconfig | 8 +
mm/Kconfig | 9 +
mm/debug_vm_pgtable.c | 4 +-
mm/huge_memory.c | 145 +++-
mm/hugetlb.c | 146 +++-
mm/internal.h | 4 +-
mm/khugepaged.c | 38 +-
mm/memory.c | 123 +++-
mm/migrate.c | 20 +-
mm/migrate_device.c | 8 +-
mm/mprotect.c | 62 +-
mm/mremap.c | 17 +-
mm/page_table_check.c | 8 +-
mm/rmap.c | 18 +-
mm/swapfile.c | 9 +-
mm/userfaultfd.c | 104 ++-
tools/include/uapi/linux/fs.h | 1 +
tools/testing/selftests/mm/uffd-unit-tests.c | 734 +++++++++++++++++++
52 files changed, 2124 insertions(+), 412 deletions(-)

--
2.51.2