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 .. _userfaultfd:
 
 ===========
 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
 ======
 
 Userfaults are delivered and resolved through the ``userfaultfd`` syscall.
 
 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 opened by invoking the syscall, 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
 ===
 
 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.
 
 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.
 
 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.
 
 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.

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