[PATCH v7 00/15] 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 hot pages and reclaim cold ones
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/
v5: https://lore.kernel.org/all/20260526130509.2748441-1-kirill@xxxxxxxxxxxxx/
v6: https://lore.kernel.org/all/20260529172716.357179-1-kas@xxxxxxxxxx/

== Changes since v6 ==

  - Rebased onto v7.2-rc1. v6 was stacked on the separate
    "userfaultfd/pagemap: pre-existing fixes" series; those fixes have
    since landed, so v7 applies directly on v7.2-rc1 with no out-of-tree
    dependency. The only rebase adaptation is that the accessor-rename
    patch now also covers two call sites in remove_migration_pmd() that
    appeared in v7.2-rc1.
  - Addressed Lorenzo Stoakes' review: the two VM_UFFD_RWP single-flag
    checks -- userfaultfd_rwp() and gup_can_follow_protnone() -- use
    vma_test_single_mask() instead of vma_test_any_mask().
  - Reworked the working-set documentation and the PAGEMAP_SCAN selftest
    around hot-page detection (PAGE_IS_ACCESSED, non-inverted) rather
    than a cold scan. A cold scan (inverted PAGE_IS_ACCESSED) cannot see
    file pages that are present in the page cache but not mapped, so it
    misreports never-faulted regions of a pre-populated file as cold.
    Tracking the hot set and reclaiming everything else from the backing
    file is the correct model and is now what the docs and tests show.
  - Documented a related file-THP limitation: RWP state is PTE-granular,
    but a file mapping faulted in as a PMD-level THP loses its RWP marks
    when the PMD is split -- split_huge_pmd() clears the file PMD via
    pmdp_huge_clear_flush() without redistributing the uffd marker to the
    PTEs (there is no PMD-level RWP marker), so the range silently reverts
    to untracked. Cold-page tracking over a transparently-huge file
    mapping is therefore unreliable; the selftest opts out of THP
    (MADV_NOHUGEPAGE) on the non-hugetlb backings, and the documented
    hot-set-plus-file-reclaim model sidesteps it. hugetlb is unaffected
    (its mappings are not split by split_huge_pmd()).
  - Collected review tags picked up during v6.

113/113 of tools/testing/selftests/mm/uffd-unit-tests pass on v7.2-rc1
(46 RWP cases plus the existing UFFD groups, no regressions).

== Problem ==

A VMM managing guest memory needs to:

  1. detect which pages are still being touched (working-set
     tracking);
  2. safely reclaim 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 during the cycle by
reading PAGE_IS_ACCESSED out of PAGEMAP_SCAN -- that set is the
working set; everything else is a reclaim candidate.

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 reclaim 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 snapshot before scanning */
  UFFDIO_SET_MODE(disable = RWP_ASYNC)                  /* sync */
  PAGEMAP_SCAN(PAGE_IS_ACCESSED) -> hot pages (working set)

  /* reclaim everything not in the hot set from the backing file */
  fallocate(FALLOC_FL_PUNCH_HOLE, non-hot) /* or pwrite to remote */
  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.

Patch 4 switches the uffd VMA-flag helpers to the vma_flags_t
accessors (vma_test_*_mask), so the VMA_UFFD_* masks are the single
place that knows which modes the build offers.

Patches 5 to 8 add the in-kernel mechanism:

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

Patches 9 to 13 wire the userspace surface:

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

Patches 14 and 15 are kernel tests and Documentation/. The matching
man-pages series is already upstream.

Kiryl Shutsemau (Meta) (15):
  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
  userfaultfd: test uffd VMA flags through the vma_flags_t API
  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 | 269 ++++++-
 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 ++-
 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                           |  65 +-
 include/linux/mm_inline.h                    |   4 +-
 include/linux/pgtable.h                      |  32 +-
 include/linux/swapops.h                      |   4 +-
 include/linux/userfaultfd_k.h                |  89 ++-
 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                             | 161 ++--
 mm/hugetlb.c                                 | 158 +++-
 mm/internal.h                                |   4 +-
 mm/khugepaged.c                              |  40 +-
 mm/memory.c                                  | 135 +++-
 mm/migrate.c                                 |  20 +-
 mm/migrate_device.c                          |   8 +-
 mm/mprotect.c                                |  70 +-
 mm/mremap.c                                  |  17 +-
 mm/page_table_check.c                        |   8 +-
 mm/rmap.c                                    |  18 +-
 mm/swapfile.c                                |   9 +-
 mm/userfaultfd.c                             | 387 ++++++++-
 tools/include/uapi/linux/fs.h                |   1 +
 tools/testing/selftests/mm/uffd-unit-tests.c | 781 +++++++++++++++++++
 51 files changed, 2333 insertions(+), 437 deletions(-)


base-commit: dc59e4fea9d83f03bad6bddf3fa2e52491777482
-- 
2.54.0