Documentation/admin-guide/mm/userfaultfd.rst

===========
Userfaultfd
===========

Objective
=========

Userfaults allow the implementation of on-demand paging from userland
and more generally they allow userland to take control of various
memory page faults, something otherwise only the kernel code could do.

For example userfaults allows a proper and more optimal implementation
of the ``PROT_NONE+SIGSEGV`` trick.

Design
======

Userspace creates a new userfaultfd, initializes it, and registers one or more
regions of virtual memory with it. Then, any page faults which occur within the
region(s) result in a message being delivered to the userfaultfd, notifying
userspace of the fault.

The ``userfaultfd`` (aside from registering and unregistering virtual
memory ranges) provides two primary functionalities:

1) ``read/POLLIN`` protocol to notify a userland thread of the faults
   happening

2) various ``UFFDIO_*`` ioctls that can manage the virtual memory regions
   registered in the ``userfaultfd`` that allows userland to efficiently
   resolve the userfaults it receives via 1) or to manage the virtual
   memory in the background

The real advantage of userfaults if compared to regular virtual memory
management of mremap/mprotect is that the userfaults in all their
operations never involve heavyweight structures like vmas (in fact the
``userfaultfd`` runtime load never takes the mmap_lock for writing).
Vmas are not suitable for page- (or hugepage) granular fault tracking
when dealing with virtual address spaces that could span
Terabytes. Too many vmas would be needed for that.

The ``userfaultfd``, once created, can also be
passed using unix domain sockets to a manager process, so the same
manager process could handle the userfaults of a multitude of
different processes without them being aware about what is going on
(well of course unless they later try to use the ``userfaultfd``
themselves on the same region the manager is already tracking, which
is a corner case that would currently return ``-EBUSY``).

API
===

Creating a userfaultfd
----------------------

There are two ways to create a new userfaultfd, each of which provide ways to
restrict access to this functionality (since historically userfaultfds which
handle kernel page faults have been a useful tool for exploiting the kernel).

The first way, supported since userfaultfd was introduced, is the
userfaultfd(2) syscall. Access to this is controlled in several ways:

- Any user can always create a userfaultfd which traps userspace page faults
  only. Such a userfaultfd can be created using the userfaultfd(2) syscall
  with the flag UFFD_USER_MODE_ONLY.

- In order to also trap kernel page faults for the address space, either the
  process needs the CAP_SYS_PTRACE capability, or the system must have
  vm.unprivileged_userfaultfd set to 1. By default, vm.unprivileged_userfaultfd
  is set to 0.

The second way, added to the kernel more recently, is by opening
/dev/userfaultfd and issuing a USERFAULTFD_IOC_NEW ioctl to it. This method
yields equivalent userfaultfds to the userfaultfd(2) syscall.

Unlike userfaultfd(2), access to /dev/userfaultfd is controlled via normal
filesystem permissions (user/group/mode), which gives fine grained access to
userfaultfd specifically, without also granting other unrelated privileges at
the same time (as e.g. granting CAP_SYS_PTRACE would do). Users who have access
to /dev/userfaultfd can always create userfaultfds that trap kernel page faults;
vm.unprivileged_userfaultfd is not considered.

Initializing a userfaultfd
--------------------------

When first opened the ``userfaultfd`` must be enabled invoking the
``UFFDIO_API`` ioctl specifying a ``uffdio_api.api`` value set to ``UFFD_API`` (or
a later API version) which will specify the ``read/POLLIN`` protocol
userland intends to speak on the ``UFFD`` and the ``uffdio_api.features``
userland requires. The ``UFFDIO_API`` ioctl if successful (i.e. if the
requested ``uffdio_api.api`` is spoken also by the running kernel and the
requested features are going to be enabled) will return into
``uffdio_api.features`` and ``uffdio_api.ioctls`` two 64bit bitmasks of
respectively all the available features of the read(2) protocol and
the generic ioctl available.

The ``uffdio_api.features`` bitmask returned by the ``UFFDIO_API`` ioctl
defines what memory types are supported by the ``userfaultfd`` and what
events, except page fault notifications, may be generated:

- The ``UFFD_FEATURE_EVENT_*`` flags indicate that various other events
  other than page faults are supported. These events are described in more
  detail below in the `Non-cooperative userfaultfd`_ section.

- ``UFFD_FEATURE_MISSING_HUGETLBFS`` and ``UFFD_FEATURE_MISSING_SHMEM``
  indicate that the kernel supports ``UFFDIO_REGISTER_MODE_MISSING``
  registrations for hugetlbfs and shared memory (covering all shmem APIs,
  i.e. tmpfs, ``IPCSHM``, ``/dev/zero``, ``MAP_SHARED``, ``memfd_create``,
  etc) virtual memory areas, respectively.

- ``UFFD_FEATURE_MINOR_HUGETLBFS`` indicates that the kernel supports
  ``UFFDIO_REGISTER_MODE_MINOR`` registration for hugetlbfs virtual memory
  areas. ``UFFD_FEATURE_MINOR_SHMEM`` is the analogous feature indicating
  support for shmem virtual memory areas.

- ``UFFD_FEATURE_MOVE`` indicates that the kernel supports moving an
  existing page contents from userspace.

The userland application should set the feature flags it intends to use
when invoking the ``UFFDIO_API`` ioctl, to request that those features be
enabled if supported.

Once the ``userfaultfd`` API has been enabled the ``UFFDIO_REGISTER``
ioctl should be invoked (if present in the returned ``uffdio_api.ioctls``
bitmask) to register a memory range in the ``userfaultfd`` by setting the
uffdio_register structure accordingly. The ``uffdio_register.mode``
bitmask will specify to the kernel which kind of faults to track for
the range. The ``UFFDIO_REGISTER`` ioctl will return the
``uffdio_register.ioctls`` bitmask of ioctls that are suitable to resolve
userfaults on the range registered. Not all ioctls will necessarily be
supported for all memory types (e.g. anonymous memory vs. shmem vs.
hugetlbfs), or all types of intercepted faults.

.. note::

   Re-registering an already-registered range must not drop any of the
   modes that install per-PTE markers — currently
   ``UFFDIO_REGISTER_MODE_WP`` and ``UFFDIO_REGISTER_MODE_RWP``. Doing
   so would strand markers with no flag to describe them, so the call
   is rejected with ``-EBUSY``; userspace must issue
   ``UFFDIO_UNREGISTER`` first. This differs from older kernels, which
   silently replaced the mode bits on re-registration.

Userland can use the ``uffdio_register.ioctls`` to manage the virtual
address space in the background (to add or potentially also remove
memory from the ``userfaultfd`` registered range). This means a userfault
could be triggering just before userland maps in the background the
user-faulted page.

Resolving Userfaults
--------------------

There are three basic ways to resolve userfaults:

- ``UFFDIO_COPY`` atomically copies some existing page contents from
  userspace.

- ``UFFDIO_ZEROPAGE`` atomically zeros the new page.

- ``UFFDIO_CONTINUE`` maps an existing, previously-populated page.

These operations are atomic in the sense that they guarantee nothing can
see a half-populated page, since readers will keep userfaulting until the
operation has finished.

By default, these wake up userfaults blocked on the range in question.
They support a ``UFFDIO_*_MODE_DONTWAKE`` ``mode`` flag, which indicates
that waking will be done separately at some later time.

Which ioctl to choose depends on the kind of page fault, and what we'd
like to do to resolve it:

- For ``UFFDIO_REGISTER_MODE_MISSING`` faults, the fault needs to be
  resolved by either providing a new page (``UFFDIO_COPY``), or mapping
  the zero page (``UFFDIO_ZEROPAGE``). By default, the kernel would map
  the zero page for a missing fault. With userfaultfd, userspace can
  decide what content to provide before the faulting thread continues.

- For ``UFFDIO_REGISTER_MODE_MINOR`` faults, there is an existing page (in
  the page cache). Userspace has the option of modifying the page's
  contents before resolving the fault. Once the contents are correct
  (modified or not), userspace asks the kernel to map the page and let the
  faulting thread continue with ``UFFDIO_CONTINUE``.

Notes:

- You can tell which kind of fault occurred by examining
  ``pagefault.flags`` within the ``uffd_msg``, checking for the
  ``UFFD_PAGEFAULT_FLAG_*`` flags.

- None of the page-delivering ioctls default to the range that you
  registered with.  You must fill in all fields for the appropriate
  ioctl struct including the range.

- You get the address of the access that triggered the missing page
  event out of a struct uffd_msg that you read in the thread from the
  uffd.  You can supply as many pages as you want with these IOCTLs.
  Keep in mind that unless you used DONTWAKE then the first of any of
  those IOCTLs wakes up the faulting thread.

- Be sure to test for all errors including
  (``pollfd[0].revents & POLLERR``).  This can happen, e.g. when ranges
  supplied were incorrect.

Write Protect Notifications
---------------------------

This is equivalent to (but faster than) using mprotect and a SIGSEGV
signal handler.

Firstly you need to register a range with ``UFFDIO_REGISTER_MODE_WP``.
Instead of using mprotect(2) you use
``ioctl(uffd, UFFDIO_WRITEPROTECT, struct *uffdio_writeprotect)``
while ``mode = UFFDIO_WRITEPROTECT_MODE_WP``
in the struct passed in.  The range does not default to and does not
have to be identical to the range you registered with.  You can write
protect as many ranges as you like (inside the registered range).
Then, in the thread reading from uffd the struct will have
``msg.arg.pagefault.flags & UFFD_PAGEFAULT_FLAG_WP`` set. Now you send
``ioctl(uffd, UFFDIO_WRITEPROTECT, struct *uffdio_writeprotect)``
again while ``pagefault.mode`` does not have ``UFFDIO_WRITEPROTECT_MODE_WP``
set. This wakes up the thread which will continue to run with writes. This
allows you to do the bookkeeping about the write in the uffd reading
thread before the ioctl.

If you registered with both ``UFFDIO_REGISTER_MODE_MISSING`` and
``UFFDIO_REGISTER_MODE_WP`` then you need to think about the sequence in
which you supply a page and undo write protect.  Note that there is a
difference between writes into a WP area and into a !WP area.  The
former will have ``UFFD_PAGEFAULT_FLAG_WP`` set, the latter
``UFFD_PAGEFAULT_FLAG_WRITE``.  The latter did not fail on protection but
you still need to supply a page when ``UFFDIO_REGISTER_MODE_MISSING`` was
used.

Userfaultfd write-protect mode currently behave differently on none ptes
(when e.g. page is missing) over different types of memories.

For anonymous memory, ``ioctl(UFFDIO_WRITEPROTECT)`` will ignore none ptes
(e.g. when pages are missing and not populated).  For file-backed memories
like shmem and hugetlbfs, none ptes will be write protected just like a
present pte.  In other words, there will be a userfaultfd write fault
message generated when writing to a missing page on file typed memories,
as long as the page range was write-protected before.  Such a message will
not be generated on anonymous memories by default.

If the application wants to be able to write protect none ptes on anonymous
memory, one can pre-populate the memory with e.g. MADV_POPULATE_READ.  On
newer kernels, one can also detect the feature UFFD_FEATURE_WP_UNPOPULATED
and set the feature bit in advance to make sure none ptes will also be
write protected even upon anonymous memory.

When using ``UFFDIO_REGISTER_MODE_WP`` in combination with either
``UFFDIO_REGISTER_MODE_MISSING`` or ``UFFDIO_REGISTER_MODE_MINOR``, when
resolving missing / minor faults with ``UFFDIO_COPY`` or ``UFFDIO_CONTINUE``
respectively, it may be desirable for the new page / mapping to be
write-protected (so future writes will also result in a WP fault). These ioctls
support a mode flag (``UFFDIO_COPY_MODE_WP`` or ``UFFDIO_CONTINUE_MODE_WP``
respectively) to configure the mapping this way.

If the userfaultfd context has ``UFFD_FEATURE_WP_ASYNC`` feature bit set,
any vma registered with write-protection will work in async mode rather
than the default sync mode.

In async mode, there will be no message generated when a write operation
happens, meanwhile the write-protection will be resolved automatically by
the kernel.  It can be seen as a more accurate version of soft-dirty
tracking and it can be different in a few ways:

  - The dirty result will not be affected by vma changes (e.g. vma
    merging) because the dirty is only tracked by the pte.

  - It supports range operations by default, so one can enable tracking on
    any range of memory as long as page aligned.

  - 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 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 the uffd bit during the procedure.

The user app can collect the "written/dirty" status by looking up the
uffd bit for the pages being interested in /proc/pagemap.

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.

When userfaultfd-wp async mode is used alone, it can be applied to all
kinds of memory.

Memory Poisioning Emulation
---------------------------

In response to a fault (either missing or minor), an action userspace can
take to "resolve" it is to issue a ``UFFDIO_POISON``. This will cause any
future faulters to either get a SIGBUS, or in KVM's case the guest will
receive an MCE as if there were hardware memory poisoning.

This is used to emulate hardware memory poisoning. Imagine a VM running on a
machine which experiences a real hardware memory error. Later, we live migrate
the VM to another physical machine. Since we want the migration to be
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.

RWP 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 architecture 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 unavailable when
the running kernel or architecture does not 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). Requesting an unsupported feature in
``uffdio_api.features`` makes ``UFFDIO_API`` fail with ``EINVAL`` and
leaves the userfaultfd context uninitialized; the structure is returned
zeroed, so the error path cannot be used to discover what the kernel
supports. The recommended probe sequence is therefore to open a
throwaway userfaultfd, call ``UFFDIO_API`` once with ``features = 0``,
inspect the returned bitmask, close that fd, then open the real one
and call ``UFFDIO_API`` again with only the supported features set.

**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 user 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()``
and calls ``vma_start_write()`` on each UFFD-armed VMA, draining
in-flight per-VMA-locked faults before the new mode takes effect.

**Working-set detection with PAGEMAP_SCAN:**

RWP-protected PTEs carry the uffd PTE bit; an access (and, in async mode, its
auto-resolution) clears it. ``PAGEMAP_SCAN`` reports ``PAGE_IS_ACCESSED`` once
the bit is clear on a ``VM_UFFD_RWP`` VMA, so a *non-inverted* scan reports the
pages that were touched during the interval -- the hot set::

    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,
        .return_mask = PAGE_IS_ACCESSED,
    };
    long n = ioctl(pagemap_fd, PAGEMAP_SCAN, &arg);

The returned ``page_region`` array lists the hot ranges. ``PAGE_IS_ACCESSED``
is set on an accessed page whether it is still present or has since been
swapped out, so the hot scan needs no ``PAGE_IS_PRESENT`` filter -- unpopulated
holes carry neither bit and are excluded on their own.

Track the hot set and reclaim everything else from the backing file (see the
workflow below). Do **not** invert the scan to enumerate "cold" pages
directly: an inverted scan reports only the ``VM_UFFD_RWP`` PTEs that are still
protected, i.e. the resident portion of *this* VMA. For a file mapping the
working set spans the whole file -- pages that live in the page cache but are
not mapped into this VMA (a pre-populated tmpfs file, or memory populated
through another mapping) are ``pte_none`` here, never appear in the scan, and
would never be considered for eviction even though they occupy memory. Driving
eviction from "file offsets minus the hot set" avoids that blind spot; a cold
PTE scan cannot. To additionally record the *first* access to a cached but
unmapped page (e.g. pre-populated content) as hot, co-register the range with
``UFFDIO_REGISTER_MODE_MINOR``: such accesses then fault as minor faults
instead of mapping the page silently.

**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. Two
mappings of the same shmem (or hugetlbfs) file are used: ``guest_mem`` is
the RWP-registered mapping that vCPUs access through, and ``io_mem`` is a
private mapping for VMM-side I/O. Reading ``io_mem`` does not go through
the RWP-protected PTEs of ``guest_mem``, so the VMM's own ``pwrite()``
never traps on its own ::

    /* One-time setup */
    fd = memfd_create("guest", MFD_CLOEXEC);
    ftruncate(fd, guest_size);
    guest_mem = mmap(NULL, guest_size, PROT_READ | PROT_WRITE,
                     MAP_SHARED, fd, 0);  /* vCPU view, RWP-registered */
    io_mem    = mmap(NULL, guest_size, PROT_READ | PROT_WRITE,
                     MAP_SHARED, fd, 0);  /* VMM I/O view, unprotected */

    uffd = userfaultfd(O_CLOEXEC | O_NONBLOCK);
    struct uffdio_api api = {
        .api = UFFD_API,
        .features = UFFD_FEATURE_RWP | UFFD_FEATURE_RWP_ASYNC,
    };
    ioctl(uffd, UFFDIO_API, &api);
    if (!(api.features & UFFD_FEATURE_RWP))
        /* RWP unavailable on this kernel/arch -- fall back. */
    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. Switch to sync BEFORE scanning. In async mode a vCPU
         * access races eviction: it would auto-resolve and mark the
         * page hot just as the VMM writes it out and punches it,
         * losing the update. Sync mode makes such accesses block and
         * be delivered, freezing the hot snapshot for the rest of the
         * iteration.
         */
        ioctl(uffd, UFFDIO_SET_MODE,
              &(struct uffdio_set_mode){
                  .disable = UFFD_FEATURE_RWP_ASYNC });

        /* 3. Read the hot set: pages touched this interval. */
        ioctl(pagemap_fd, PAGEMAP_SCAN, &(struct pm_scan_arg){
            .category_mask = PAGE_IS_ACCESSED,
            .return_mask = PAGE_IS_ACCESSED,
            ...
        });

        /*
         * 4. Reclaim the file offsets that are NOT in the hot set.
         * Driving this from the file's offset space (rather than from a
         * cold PTE scan) also reclaims pages that are cached but not
         * mapped into guest_mem, e.g. pre-populated content.
         */
        for each non-hot offset range:
            /* Read from io_mem -- bypasses RWP, no fault. */
            pwrite(storage_fd, (char *)io_mem + off, len, off);
            /* Drop the page from the shared file. */
            fallocate(fd, FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE,
                      off, len);
            /*
             * Wake any vCPU blocked on the RWP fault for this range:
             * fallocate() does not iterate ctx->fault_pending_wqh.
             */
            ioctl(uffd, UFFDIO_WAKE, &(struct uffdio_range){
                .start = (uintptr_t)guest_mem + off, .len = len });

        /* 5. Resume async tracking */
        ioctl(uffd, UFFDIO_SET_MODE,
              &(struct uffdio_set_mode){
                  .enable = UFFD_FEATURE_RWP_ASYNC });
    }

During step 4, a vCPU that accesses a ``guest_mem`` offset being evicted
blocks with a ``UFFD_PAGEFAULT_FLAG_RWP`` fault while the eviction is in
progress. After ``fallocate()`` punches the page out and ``UFFDIO_WAKE``
fires, the vCPU retries the access, faults as ``MISSING``, and the
handler resolves it with ``UFFDIO_COPY`` from storage.

This workflow targets shmem and hugetlbfs (both support a private
``io_mem`` mapping over the same fd). Anonymous-memory backings need a
different inner-loop strategy because the VMM has no way to read the
page without going through the RWP-protected mapping.

QEMU/KVM
========

QEMU/KVM is using the ``userfaultfd`` syscall to implement postcopy live
migration. Postcopy live migration is one form of memory
externalization consisting of a virtual machine running with part or
all of its memory residing on a different node in the cloud. The
``userfaultfd`` abstraction is generic enough that not a single line of
KVM kernel code had to be modified in order to add postcopy live
migration to QEMU.

Guest async page faults, ``FOLL_NOWAIT`` and all other ``GUP*`` features work
just fine in combination with userfaults. Userfaults trigger async
page faults in the guest scheduler so those guest processes that
aren't waiting for userfaults (i.e. network bound) can keep running in
the guest vcpus.

It is generally beneficial to run one pass of precopy live migration
just before starting postcopy live migration, in order to avoid
generating userfaults for readonly guest regions.

The implementation of postcopy live migration currently uses one
single bidirectional socket but in the future two different sockets
will be used (to reduce the latency of the userfaults to the minimum
possible without having to decrease ``/proc/sys/net/ipv4/tcp_wmem``).

The QEMU in the source node writes all pages that it knows are missing
in the destination node, into the socket, and the migration thread of
the QEMU running in the destination node runs ``UFFDIO_COPY|ZEROPAGE``
ioctls on the ``userfaultfd`` in order to map the received pages into the
guest (``UFFDIO_ZEROCOPY`` is used if the source page was a zero page).

A different postcopy thread in the destination node listens with
poll() to the ``userfaultfd`` in parallel. When a ``POLLIN`` event is
generated after a userfault triggers, the postcopy thread read() from
the ``userfaultfd`` and receives the fault address (or ``-EAGAIN`` in case the
userfault was already resolved and waken by a ``UFFDIO_COPY|ZEROPAGE`` run
by the parallel QEMU migration thread).

After the QEMU postcopy thread (running in the destination node) gets
the userfault address it writes the information about the missing page
into the socket. The QEMU source node receives the information and
roughly "seeks" to that page address and continues sending all
remaining missing pages from that new page offset. Soon after that
(just the time to flush the tcp_wmem queue through the network) the
migration thread in the QEMU running in the destination node will
receive the page that triggered the userfault and it'll map it as
usual with the ``UFFDIO_COPY|ZEROPAGE`` (without actually knowing if it
was spontaneously sent by the source or if it was an urgent page
requested through a userfault).

By the time the userfaults start, the QEMU in the destination node
doesn't need to keep any per-page state bitmap relative to the live
migration around and a single per-page bitmap has to be maintained in
the QEMU running in the source node to know which pages are still
missing in the destination node. The bitmap in the source node is
checked to find which missing pages to send in round robin and we seek
over it when receiving incoming userfaults. After sending each page of
course the bitmap is updated accordingly. It's also useful to avoid
sending the same page twice (in case the userfault is read by the
postcopy thread just before ``UFFDIO_COPY|ZEROPAGE`` runs in the migration
thread).

Non-cooperative userfaultfd
===========================

When the ``userfaultfd`` is monitored by an external manager, the manager
must be able to track changes in the process virtual memory
layout. Userfaultfd can notify the manager about such changes using
the same read(2) protocol as for the page fault notifications. The
manager has to explicitly enable these events by setting appropriate
bits in ``uffdio_api.features`` passed to ``UFFDIO_API`` ioctl:

``UFFD_FEATURE_EVENT_FORK``
	enable ``userfaultfd`` hooks for fork(). When this feature is
	enabled, the ``userfaultfd`` context of the parent process is
	duplicated into the newly created process. The manager
	receives ``UFFD_EVENT_FORK`` with file descriptor of the new
	``userfaultfd`` context in the ``uffd_msg.fork``.

``UFFD_FEATURE_EVENT_REMAP``
	enable notifications about mremap() calls. When the
	non-cooperative process moves a virtual memory area to a
	different location, the manager will receive
	``UFFD_EVENT_REMAP``. The ``uffd_msg.remap`` will contain the old and
	new addresses of the area and its original length.

``UFFD_FEATURE_EVENT_REMOVE``
	enable notifications about madvise(MADV_REMOVE) and
	madvise(MADV_DONTNEED) calls. The event ``UFFD_EVENT_REMOVE`` will
	be generated upon these calls to madvise(). The ``uffd_msg.remove``
	will contain start and end addresses of the removed area.

``UFFD_FEATURE_EVENT_UNMAP``
	enable notifications about memory unmapping. The manager will
	get ``UFFD_EVENT_UNMAP`` with ``uffd_msg.remove`` containing start and
	end addresses of the unmapped area.

Although the ``UFFD_FEATURE_EVENT_REMOVE`` and ``UFFD_FEATURE_EVENT_UNMAP``
are pretty similar, they quite differ in the action expected from the
``userfaultfd`` manager. In the former case, the virtual memory is
removed, but the area is not, the area remains monitored by the
``userfaultfd``, and if a page fault occurs in that area it will be
delivered to the manager. The proper resolution for such page fault is
to zeromap the faulting address. However, in the latter case, when an
area is unmapped, either explicitly (with munmap() system call), or
implicitly (e.g. during mremap()), the area is removed and in turn the
``userfaultfd`` context for such area disappears too and the manager will
not get further userland page faults from the removed area. Still, the
notification is required in order to prevent manager from using
``UFFDIO_COPY`` on the unmapped area.

Unlike userland page faults which have to be synchronous and require
explicit or implicit wakeup, all the events are delivered
asynchronously and the non-cooperative process resumes execution as
soon as manager executes read(). The ``userfaultfd`` manager should
carefully synchronize calls to ``UFFDIO_COPY`` with the events
processing. To aid the synchronization, the ``UFFDIO_COPY`` ioctl will
return ``-ENOSPC`` when the monitored process exits at the time of
``UFFDIO_COPY``, and ``-ENOENT``, when the non-cooperative process has changed
its virtual memory layout simultaneously with outstanding ``UFFDIO_COPY``
operation.

The current asynchronous model of the event delivery is optimal for
single threaded non-cooperative ``userfaultfd`` manager implementations. A
synchronous event delivery model can be added later as a new
``userfaultfd`` feature to facilitate multithreading enhancements of the
non cooperative manager, for example to allow ``UFFDIO_COPY`` ioctls to
run in parallel to the event reception. Single threaded
implementations should continue to use the current async event
delivery model instead.