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

["Kiryl Shutsemau (Meta)" <kas@kernel.org>]

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