Add an admin-guide section covering UFFDIO_REGISTER_MODE_RWP: - sync and async fault models; - UFFDIO_RWPROTECT semantics; - UFFD_FEATURE_RWP_ASYNC; - UFFDIO_SET_MODE runtime mode flips.
It also covers typical VMM working-set-tracking workflow from detection loop through sync-mode eviction and back to async. Signed-off-by: Kiryl Shutsemau <[email protected]> Assisted-by: Claude:claude-opus-4-6 --- Documentation/admin-guide/mm/userfaultfd.rst | 201 ++++++++++++++++++- 1 file changed, 195 insertions(+), 6 deletions(-) diff --git a/Documentation/admin-guide/mm/userfaultfd.rst b/Documentation/admin-guide/mm/userfaultfd.rst index 1e533639fd50..c6304ddcf238 100644 --- a/Documentation/admin-guide/mm/userfaultfd.rst +++ b/Documentation/admin-guide/mm/userfaultfd.rst @@ -275,16 +275,16 @@ tracking and it can be different in a few ways: - Dirty information will not get lost if the pte was zapped due to various reasons (e.g. during split of a shmem transparent huge page). - - Due to a reverted meaning of soft-dirty (page clean when uffd-wp bit - set; dirty when uffd-wp bit cleared), it has different semantics on - some of the memory operations. For example: ``MADV_DONTNEED`` on + - Due to a reverted meaning of soft-dirty (page clean when the uffd bit + is set; dirty when the uffd bit is cleared), it has different semantics + on some of the memory operations. For example: ``MADV_DONTNEED`` on anonymous (or ``MADV_REMOVE`` on a file mapping) will be treated as - dirtying of memory by dropping uffd-wp bit during the procedure. + dirtying of memory by dropping the uffd bit during the procedure. The user app can collect the "written/dirty" status by looking up the -uffd-wp bit for the pages being interested in /proc/pagemap. +uffd bit for the pages being interested in /proc/pagemap. -The page will not be under track of uffd-wp async mode until the page is +The page will not be under track of userfaultfd-wp async mode until the page is explicitly write-protected by ``ioctl(UFFDIO_WRITEPROTECT)`` with the mode flag ``UFFDIO_WRITEPROTECT_MODE_WP`` set. Trying to resolve a page fault that was tracked by async mode userfaultfd-wp is invalid. @@ -307,6 +307,195 @@ transparent to the guest, we want that same address range to act as if it was still poisoned, even though it's on a new physical host which ostensibly doesn't have a memory error in the exact same spot. +Read-Write Protection +--------------------- + +``UFFDIO_REGISTER_MODE_RWP`` enables read-write protection tracking on a +memory range. It is similar to (but faster than) ``mprotect(PROT_NONE)`` +combined with a signal handler; unlike ``mprotect(PROT_NONE)``, RWP only +traps accesses to *present* PTEs, so accesses to unpopulated addresses in a +protected range fall through to the normal missing-page path. It uses the +PROT_NONE hinting mechanism (same as NUMA balancing) to make pages +inaccessible while keeping them resident in memory. Works on anonymous, +shmem, and hugetlbfs memory. + +This is designed for VM memory managers that need to track the working set +of guest memory for cold page eviction to tiered or remote storage. + +**Setup:** + +1. Open a userfaultfd and enable ``UFFD_FEATURE_RWP`` via ``UFFDIO_API``. + Optionally request ``UFFD_FEATURE_RWP_ASYNC`` as well — it requires + ``UFFD_FEATURE_RWP`` to be set in the same ``UFFDIO_API`` call. + +2. Register the guest memory range with ``UFFDIO_REGISTER_MODE_RWP`` + (and ``UFFDIO_REGISTER_MODE_MISSING`` if evicted pages will need to be + fetched back from storage). + +**Feature availability:** + +RWP is built on top of two kernel primitives: a spare PTE bit owned by +userfaultfd (``CONFIG_HAVE_ARCH_USERFAULTFD_WP``) and arch support for +present-but-inaccessible PTEs (``CONFIG_ARCH_HAS_PTE_PROTNONE``). When both +are available on a 64-bit kernel, the build selects +``CONFIG_USERFAULTFD_RWP=y`` and the ``VM_UFFD_RWP`` VMA flag becomes +available. + +``UFFD_FEATURE_RWP`` and ``UFFD_FEATURE_RWP_ASYNC`` are masked out of the +features returned by ``UFFDIO_API`` when the running kernel or architecture +cannot support them — for example 32-bit kernels (where ``VM_UFFD_RWP`` is +unavailable), kernels built without ``CONFIG_USERFAULTFD_RWP``, and +architectures whose ptes cannot carry the uffd bit at runtime (e.g. riscv +without the ``SVRSW60T59B`` extension). ``UFFDIO_API`` does not fail; +unsupported bits are simply absent from ``uffdio_api.features`` on return. +VMMs should inspect the returned ``features`` after ``UFFDIO_API`` and fall +back to another tracking method when RWP is unavailable. + +**Protecting and Unprotecting:** + +Use ``UFFDIO_RWPROTECT`` to protect or unprotect a range, mirroring the +``UFFDIO_WRITEPROTECT`` interface:: + + struct uffdio_rwprotect rwp = { + .range = { .start = addr, .len = len }, + .mode = UFFDIO_RWPROTECT_MODE_RWP, /* protect */ + }; + ioctl(uffd, UFFDIO_RWPROTECT, &rwp); + +Setting ``UFFDIO_RWPROTECT_MODE_RWP`` sets PROT_NONE on present PTEs in the +range. Pages stay resident and their physical frames are preserved — only +access permissions are removed. + +Clearing ``UFFDIO_RWPROTECT_MODE_RWP`` restores normal VMA permissions and +wakes any faulting threads (unless ``UFFDIO_RWPROTECT_MODE_DONTWAKE`` is set). + +**Scope of protection:** + +RWP protection is a property of *present* PTEs. ``UFFDIO_RWPROTECT`` only +affects entries that are already populated. Unpopulated addresses within +the range remain unpopulated; when first accessed they fault through the +normal missing path (``do_anonymous_page()``, ``do_swap_page()``, +``finish_fault()``) and the resulting PTE is not RWP-protected. To observe +the population itself, co-register the range with +``UFFDIO_REGISTER_MODE_MISSING``. + +Protection is preserved across page reclaim: a page swapped out while +RWP-protected carries the marker on its swap entry, and swap-in restores +the PROT_NONE state so the first access after swap-in still faults. The +same applies to pages temporarily replaced by migration entries. + +Operations that drop the PTE entirely — ``MADV_DONTNEED`` on anonymous +memory, hole-punch on shmem, truncation of a file mapping — also drop the +RWP marker: the next access re-populates the range without protection. +Unlike WP (which persists via ``PTE_MARKER_UFFD_WP``), there is no +persistent RWP marker today. The VMM needs to re-arm the range with +``UFFDIO_RWPROTECT`` after any operation that explicitly frees PTEs. + +**Fault Handling:** + +When a protected page is accessed: + +- **Sync mode** (default): The faulting thread blocks and a + ``UFFD_PAGEFAULT_FLAG_RWP`` message is delivered to the userfaultfd + handler. The handler resolves the fault with ``UFFDIO_RWPROTECT`` + (clearing ``MODE_RWP``), which restores the PTE permissions and wakes + the faulting thread. + +- **Async mode** (``UFFD_FEATURE_RWP_ASYNC``): The kernel automatically + restores PTE permissions and the thread continues without blocking. No + message is delivered to the handler. + +**Runtime Mode Switching:** + +``UFFDIO_SET_MODE`` toggles ``UFFD_FEATURE_RWP_ASYNC`` at runtime, allowing +the VMM to switch between lightweight async detection and safe sync +eviction without re-registering. The toggle takes ``mmap_write_lock()`` to +ensure all in-flight faults complete before the mode change takes effect. + +**Cold Page Detection with PAGEMAP_SCAN:** + +RWP-protected PTEs carry the uffd PTE bit; the fault-resolution path +clears it. ``PAGEMAP_SCAN`` reports ``PAGE_IS_ACCESSED`` once the bit is +clear on a ``VM_UFFD_RWP`` VMA, so inverting it efficiently reports the +still-protected (cold) pages:: + + struct pm_scan_arg arg = { + .size = sizeof(arg), + .start = guest_mem_start, + .end = guest_mem_end, + .vec = (uint64_t)regions, + .vec_len = regions_len, + .category_mask = PAGE_IS_ACCESSED, + .category_inverted = PAGE_IS_ACCESSED, + .return_mask = PAGE_IS_ACCESSED, + }; + long n = ioctl(pagemap_fd, PAGEMAP_SCAN, &arg); + +The returned ``page_region`` array contains contiguous cold ranges that can +then be evicted. + +**Cleanup:** + +When the userfaultfd is closed or the range is unregistered, all PROT_NONE +PTEs are automatically restored to their normal VMA permissions. This +prevents pages from becoming permanently inaccessible. + +**VMM Working Set Tracking Workflow:** + +A typical VMM lifecycle for cold page eviction to tiered storage:: + + /* One-time setup */ + uffd = userfaultfd(O_CLOEXEC | O_NONBLOCK); + ioctl(uffd, UFFDIO_API, &(struct uffdio_api){ + .api = UFFD_API, + .features = UFFD_FEATURE_RWP | UFFD_FEATURE_RWP_ASYNC, + }); + ioctl(uffd, UFFDIO_REGISTER, &(struct uffdio_register){ + .range = { guest_mem, guest_size }, + .mode = UFFDIO_REGISTER_MODE_RWP | + UFFDIO_REGISTER_MODE_MISSING, + }); + + /* Tracking loop */ + while (vm_running) { + /* 1. Detection phase (async — no vCPU stalls) */ + ioctl(uffd, UFFDIO_RWPROTECT, &(struct uffdio_rwprotect){ + .range = full_range, + .mode = UFFDIO_RWPROTECT_MODE_RWP }); + sleep(tracking_interval); + + /* 2. Find cold pages (uffd bit still set) */ + ioctl(pagemap_fd, PAGEMAP_SCAN, &(struct pm_scan_arg){ + .category_mask = PAGE_IS_ACCESSED, + .category_inverted = PAGE_IS_ACCESSED, + .return_mask = PAGE_IS_ACCESSED, + ... + }); + + /* 3. Switch to sync for safe eviction */ + ioctl(uffd, UFFDIO_SET_MODE, + &(struct uffdio_set_mode){ + .disable = UFFD_FEATURE_RWP_ASYNC }); + + /* 4. Evict cold pages (vCPU faults block in handler) */ + for each cold range: + pwrite(storage_fd, cold_addr, len, offset); + madvise(cold_addr, len, MADV_DONTNEED); + + /* 5. Resume async tracking */ + ioctl(uffd, UFFDIO_SET_MODE, + &(struct uffdio_set_mode){ + .enable = UFFD_FEATURE_RWP_ASYNC }); + } + +During step 4, if a vCPU accesses a cold page being evicted, it blocks +with a ``UFFD_PAGEFAULT_FLAG_RWP`` fault. The handler can either let it +wait (the eviction completes, ``MADV_DONTNEED`` fires, the fault retries as +``MISSING`` and is resolved with ``UFFDIO_COPY`` from storage) or unprotect +it immediately with ``UFFDIO_RWPROTECT``. + +This workflow works identically for anonymous, shmem, and hugetlbfs memory. + QEMU/KVM ======== -- 2.51.2
