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/[email protected]/ == Changes since v1 == Review feedback from Mike Rapoport, SeongJae Park, and the sashiko AI review (https://sashiko.dev/#/patchset/[email protected]). Per-patch: - 01/14 (decouple protnone): rephrased the !ARCH_HAS_PTE_PROTNONE comment to keep the original pte_protnone() semantics description (Mike Rapoport). Acked-by Mike Rapoport, SeongJae Park. - 02/14 (rename uffd-wp PTE bit macros): Reviewed-by Mike Rapoport. - 03/14 (rename uffd-wp PTE accessors): Reviewed-by Mike Rapoport. - 04/14 (VM_UFFD_RWP VMA flag): __VMA_UFFD_FLAGS now includes VMA_UFFD_RWP_BIT so RWP deregistration cleanly merges adjacent non-uffd VMAs. The VM_COPY_ON_FORK note no longer singles out VM_UFFD_WP (sashiko). - 06/14 (preserve RWP marker): __copy_present_ptes() snapshots pte_write() before the RWP-disarm pte_modify(), and the COW wrprotect uses the snapshot. Without it a fork() without UFFD_FEATURE_EVENT_FORK could leave the parent writable over a folio shared with the child. hugetlb_install_folio() (the pinned-fork hugetlb fallback) now uses userfaultfd_protected() and applies PAGE_NONE on userfaultfd_rwp(vma), mirroring copy_present_page() (sashiko). - 08/14 (UFFDIO_REGISTER_MODE_RWP plumbing): MM_CP_TRY_CHANGE_WRITABLE is set per-VMA inside the iteration loop, gated on vma_wants_manual_pte_write_upgrade(). RWP register accepts PROT_READ-only mappings, so the flat outer flag would have tripped the WARN_ON_ONCE in maybe_change_pte_writable() on resolve (sashiko). - 10/14 (PAGE_IS_ACCESSED in PAGEMAP_SCAN): pagemap_scan_test_walk() now returns -EINVAL when PM_SCAN_WP_MATCHING is set on a VM_UFFD_RWP VMA, instead of silently skipping the range (sashiko). - 12/14 (UFFDIO_SET_MODE): added userfaultfd_features() helper wrapping READ_ONCE(ctx->features); converted lockless readers (userfaultfd_is_initialized, userfaultfd_wp_async_ctx, userfaultfd_rwp_async_ctx, userfaultfd_wp_unpopulated, fdinfo). Hot-path fault-handler reads stay plain since the SET_MODE drain excludes them (sashiko). - 13/14 (selftests): rwp-sync and rwp-async-toggle tests join the fault-handler thread before reading the minor_faults counter, so the last fault's increment is always visible. The async-toggle test stops the handler between Phase 2 and Phase 3 so a regression that erroneously delivers a sync fault in async mode is no longer silently masked. rwp-fork-pin now requires UFFD_FEATURE_EVENT_FORK (and runs a fork_event_consumer), so the child genuinely inherits the marker; otherwise userfaultfd_reset_ctx() would clear it and the test would pass for the wrong reason. rwp-wp-exclusive now requires UFFD_FEATURE_WP_HUGETLBFS_SHMEM so it skips cleanly on kernels without WP-marker support for shmem/hugetlbfs. Tightened the GUP test's pipe write down to a single byte. Stale "WP and RWP coexisting" comment removed (sashiko). - 14/14 (Documentation): VMM workflow rewritten to use a second mapping of the same memfd for VMM-side I/O, so pwrite() does not fault on the protnone-protected PTE. madvise(MADV_DONTNEED) replaced with fallocate(FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE) -- DONTNEED only zaps PTEs and does not free shmem pages. Added explicit UFFDIO_WAKE after fallocate() since neither PUNCH_HOLE nor DONTNEED iterates ctx->fault_pending_wqh (sashiko). == 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. 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 | 236 +++++- 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 | 108 ++- fs/userfaultfd.c | 264 ++++++- 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 | 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 | 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 | 113 ++- tools/include/uapi/linux/fs.h | 1 + tools/testing/selftests/mm/uffd-unit-tests.c | 774 +++++++++++++++++++ 52 files changed, 2235 insertions(+), 413 deletions(-) base-commit: 254f49634ee16a731174d2ae34bc50bd5f45e731 -- 2.51.2
