[PATCH v5 00/18] userfaultfd: working set tracking for VM guest memory

[Kiryl Shutsemau]

From: "Kiryl Shutsemau (Meta)" <kas@xxxxxxxxxx>

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.

v1: https://lore.kernel.org/all/20260427114607.4068647-1-kas@xxxxxxxxxx/
v2: https://lore.kernel.org/all/cover.1778254670.git.kas@xxxxxxxxxx/
v3: https://lore.kernel.org/all/20260522133857.552279-1-kirill@xxxxxxxxxxxxx/
v4: https://lore.kernel.org/all/20260525113737.1942478-1-kas@xxxxxxxxxx/

== Changes since v4 ==

v5 mainly addresses Sashiko AI review feedback on v4.

  - Patches 1-4 are new pre-existing fixes surfaced by that review
    (each carries Fixes:/Cc: stable@).
  - 05/18: LoongArch select of ARCH_HAS_PTE_PROTNONE gated on 64BIT.
  - 10/18, 13/18: gate RWP disarm/rebuild paths on pte_uffd() so
    NUMA-balancing PROT_NONE markers survive.
  - 12/18: reject UFFDIO_REGISTER_MODE_RWP on PROT_NONE VMAs.
  - 14/18: PM_SCAN_WP_MATCHING on a VM_UFFD_RWP VMA silently skips
    instead of -EINVAL, preserving the atomic read-and-reset.
  - 16/18: UFFDIO_SET_MODE feature check goes through
    userfaultfd_features() for KCSAN clean read.
  - 17/18: drop _UFFDIO_SET_MODE from the baseline UFFDIO_API check
    so the test still passes on older kernels.
  - 18/18: VMM example switches to sync before PAGEMAP_SCAN; names
    uffdio_api so callers can read back negotiated features; fixes
    a couple of stray identifiers in the eviction loop.

113/113 of tools/testing/selftests/mm/uffd-unit-tests pass (46 new
RWP cases + existing UFFD groups, no regressions from patches 1-4).

== 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() + vma_start_write(), 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 (async) */
  UFFDIO_RWPROTECT(range, RWP)
  sleep(interval)

  /* freeze the cold snapshot before scanning */
  UFFDIO_SET_MODE(disable = RWP_ASYNC)                  /* sync */
  PAGEMAP_SCAN(!PAGE_IS_ACCESSED) -> cold pages

  /* eviction (sync mode traps races) */
  pwrite(cold) + fallocate(FALLOC_FL_PUNCH_HOLE, cold)
  UFFDIO_WAKE(cold)
  UFFDIO_SET_MODE(enable  = RWP_ASYNC)                  /* resume */

== Series layout ==

Patches 1 to 4 are independent pre-existing fixes (Fixes:/Cc: stable@)
that the RWP code shares paths with -- they can be picked separately
if needed:

  1: fs/proc/task_mmu: huge make_uffd_wp_huge_pte() prot-update race
     -- missing huge_ptep_modify_prot_start() can lose hardware
     Dirty/Accessed updates.
  2: mm/huge_memory: change_non_present_huge_pmd() drops
     pmd_swp_uffd_wp on the writable -> readable device-private
     PMD rewrite; plain mprotect() silently strips the marker.
  3: userfaultfd: must_wait() applies pte_write() to a locklessly
     read PTE without checking pte_present() -- swap/migration
     entries decode random offset bits and the thread can stay
     parked on a stale fault.
  4: mm: mk_vma_flags() OOBs into the first word of vma_flags_t on
     32-bit when called with a bit >= BITS_PER_LONG. Harmless by
     coincidence today (the wraparound lands on a bit that's
     already in the mask), but any future high-numbered bit would
     silently corrupt the result. Add VMA_NO_BIT and skip negative
     bits in DECLARE_VMA_BIT().

Patches 5 to 7 are preparatory:

  5: decouple protnone helpers from CONFIG_NUMA_BALANCING.
  6-7: 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 8 to 11 add the in-kernel mechanism:

  8: VM_UFFD_RWP VMA flag (aliased to VM_NONE until 12/18 introduces
     CONFIG_USERFAULTFD_RWP together with the UAPI).
  9: MM_CP_UFFD_RWP change_protection() primitive (PAGE_NONE +
     uffd bit, plus a RESOLVE counterpart).
 10: marker preservation across swap, device-exclusive, migration,
     fork, mremap, UFFDIO_MOVE, hugetlb copy, and mprotect().
 11: handle VM_UFFD_RWP in khugepaged, rmap, and GUP.

Patches 12 to 16 wire the userspace surface:

  12: UFFDIO_REGISTER_MODE_RWP and UFFDIO_RWPROTECT plumbing
      (introduces CONFIG_USERFAULTFD_RWP).
  13: RWP fault delivery and exposure of UFFDIO_REGISTER_MODE_RWP.
  14: PAGE_IS_ACCESSED in PAGEMAP_SCAN.
  15: UFFD_FEATURE_RWP_ASYNC for async fault resolution.
  16: UFFDIO_SET_MODE for runtime sync/async toggle.

Patches 17 and 18 are kernel tests and Documentation/. Matching
userfaultfd(2) and ioctl_userfaultfd(2) man-page updates will be
sent as a separate patchset against the kernel.org linux-man tree.

Kiryl Shutsemau (Meta) (18):
  fs/proc/task_mmu: fix make_uffd_wp_huge_pte() prot-update race
  mm/huge_memory: preserve pmd_swp_uffd_wp on device-private PMD
    downgrade
  userfaultfd: gate must_wait writability check on pte_present()
  mm: skip out-of-range bits in mk_vma_flags()
  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 | 248 +++++-
 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                           | 120 ++-
 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                           |  61 +-
 include/linux/mm_inline.h                    |   4 +-
 include/linux/pgtable.h                      |  32 +-
 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                             | 157 +++-
 mm/hugetlb.c                                 | 158 +++-
 mm/internal.h                                |   4 +-
 mm/khugepaged.c                              |  40 +-
 mm/memory.c                                  | 133 +++-
 mm/migrate.c                                 |  20 +-
 mm/migrate_device.c                          |   8 +-
 mm/mprotect.c                                |  68 +-
 mm/mremap.c                                  |  17 +-
 mm/page_table_check.c                        |   8 +-
 mm/rmap.c                                    |  18 +-
 mm/swapfile.c                                |   9 +-
 mm/userfaultfd.c                             | 407 +++++++++-
 tools/include/uapi/linux/fs.h                |   1 +
 tools/testing/selftests/mm/uffd-unit-tests.c | 765 +++++++++++++++++++
 51 files changed, 2321 insertions(+), 429 deletions(-)


base-commit: 449a5df98f8dffa9b037e3b6838fc5af327df072
-- 
2.54.0