admin-guide/mm/userfaultfd.rst

10 Userfaults allow the implementation of on-demand paging from userland
12 memory page faults, something otherwise only the kernel code could do.
15 of the ``PROT_NONE+SIGSEGV`` trick.
20 Userfaults are delivered and resolved through the ``userfaultfd`` syscall.
22 The ``userfaultfd`` (aside from registering and unregistering virtual
25 1) ``read/POLLIN`` protocol to notify a userland thread of the faults
28 2) various ``UFFDIO_*`` ioctls that can manage the virtual memory regions
29    registered in the ``userfaultfd`` that allows userland to efficiently
30    resolve the userfaults it receives via 1) or to manage the virtual
31    memory in the background
33 The real advantage of userfaults if compared to regular virtual memory
34 management of mremap/mprotect is that the userfaults in all their
35 operations never involve heavyweight structures like vmas (in fact the
36 ``userfaultfd`` runtime load never takes the mmap_lock for writing).
42 The ``userfaultfd`` once opened by invoking the syscall, can also be
43 passed using unix domain sockets to a manager process, so the same
44 manager process could handle the userfaults of a multitude of
46 (well of course unless they later try to use the ``userfaultfd``
47 themselves on the same region the manager is already tracking, which
53 When first opened the ``userfaultfd`` must be enabled invoking the
55 a later API version) which will specify the ``read/POLLIN`` protocol
56 userland intends to speak on the ``UFFD`` and the ``uffdio_api.features``
57 userland requires. The ``UFFDIO_API`` ioctl if successful (i.e. if the
58 requested ``uffdio_api.api`` is spoken also by the running kernel and the
61 respectively all the available features of the read(2) protocol and
62 the generic ioctl available.
64 The ``uffdio_api.features`` bitmask returned by the ``UFFDIO_API`` ioctl
65 defines what memory types are supported by the ``userfaultfd`` and what
68 If the kernel supports registering ``userfaultfd`` ranges on hugetlbfs
71 set if the kernel supports registering ``userfaultfd`` ranges on shared
75 The userland application that wants to use ``userfaultfd`` with hugetlbfs
76 or shared memory need to set the corresponding flag in
79 If the userland desires to receive notifications for events other than
84 Once the ``userfaultfd`` has been enabled the ``UFFDIO_REGISTER`` ioctl should
85 be invoked (if present in the returned ``uffdio_api.ioctls`` bitmask) to
86 register a memory range in the ``userfaultfd`` by setting the
87 uffdio_register structure accordingly. The ``uffdio_register.mode``
88 bitmask will specify to the kernel which kind of faults to track for
89 the range (``UFFDIO_REGISTER_MODE_MISSING`` would track missing
90 pages). The ``UFFDIO_REGISTER`` ioctl will return the
92 userfaults on the range registered. Not all ioctls will necessarily be
93 supported for all memory types depending on the underlying virtual
97 Userland can use the ``uffdio_register.ioctls`` to manage the virtual
98 address space in the background (to add or potentially also remove
99 memory from the ``userfaultfd`` registered range). This means a userfault
100 could be triggering just before userland maps in the background the
103 The primary ioctl to resolve userfaults is ``UFFDIO_COPY``. That
104 atomically copies a page into the userfault registered range and wakes
105 up the blocked userfaults
109 keep userfaulting until the copy has finished.
115   the uffd.  You must provide either ``UFFDIO_COPY`` or ``UFFDIO_ZEROPAGE``.
116   The normal behavior of the OS automatically providing a zero page on
119 - None of the page-delivering ioctls default to the range that you
120   registered with.  You must fill in all fields for the appropriate
121   ioctl struct including the range.
123 - You get the address of the access that triggered the missing page
124   event out of a struct uffd_msg that you read in the thread from the
127   the first of any of those IOCTLs wakes up the faulting thread.
143 in the struct passed in.  The range does not default to and does not
144 have to be identical to the range you registered with.  You can write
145 protect as many ranges as you like (inside the registered range).
146 Then, in the thread reading from uffd the struct will have
150 set. This wakes up the thread which will continue to run with writes. This
151 allows you to do the bookkeeping about the write in the uffd reading
152 thread before the ioctl.
155 ``UFFDIO_REGISTER_MODE_WP`` then you need to think about the sequence in
157 difference between writes into a WP area and into a !WP area.  The
158 former will have ``UFFD_PAGEFAULT_FLAG_WP`` set, the latter
159 ``UFFD_PAGEFAULT_FLAG_WRITE``.  The latter did not fail on protection but
166 QEMU/KVM is using the ``userfaultfd`` syscall to implement postcopy live
169 all of its memory residing on a different node in the cloud. The
176 page faults in the guest scheduler so those guest processes that
178 the guest vcpus.
184 The implementation of postcopy live migration currently uses one
185 single bidirectional socket but in the future two different sockets
186 will be used (to reduce the latency of the userfaults to the minimum
189 The QEMU in the source node writes all pages that it knows are missing
190 in the destination node, into the socket, and the migration thread of
191 the QEMU running in the destination node runs ``UFFDIO_COPY|ZEROPAGE``
192 ioctls on the ``userfaultfd`` in order to map the received pages into the
193 guest (``UFFDIO_ZEROCOPY`` is used if the source page was a zero page).
195 A different postcopy thread in the destination node listens with
196 poll() to the ``userfaultfd`` in parallel. When a ``POLLIN`` event is
197 generated after a userfault triggers, the postcopy thread read() from
198 the ``userfaultfd`` and receives the fault address (or ``-EAGAIN`` in case the
200 by the parallel QEMU migration thread).
202 After the QEMU postcopy thread (running in the destination node) gets
203 the userfault address it writes the information about the missing page
204 into the socket. The QEMU source node receives the information and
207 (just the time to flush the tcp_wmem queue through the network) the
208 migration thread in the QEMU running in the destination node will
209 receive the page that triggered the userfault and it'll map it as
210 usual with the ``UFFDIO_COPY|ZEROPAGE`` (without actually knowing if it
211 was spontaneously sent by the source or if it was an urgent page
214 By the time the userfaults start, the QEMU in the destination node
215 doesn't need to keep any per-page state bitmap relative to the live
217 the QEMU running in the source node to know which pages are still
218 missing in the destination node. The bitmap in the source node is
221 course the bitmap is updated accordingly. It's also useful to avoid
222 sending the same page twice (in case the userfault is read by the
223 postcopy thread just before ``UFFDIO_COPY|ZEROPAGE`` runs in the migration
229 When the ``userfaultfd`` is monitored by an external manager, the manager
230 must be able to track changes in the process virtual memory
231 layout. Userfaultfd can notify the manager about such changes using
232 the same read(2) protocol as for the page fault notifications. The
238 	enabled, the ``userfaultfd`` context of the parent process is
239 	duplicated into the newly created process. The manager
240 	receives ``UFFD_EVENT_FORK`` with file descriptor of the new
241 	``userfaultfd`` context in the ``uffd_msg.fork``.
244 	enable notifications about mremap() calls. When the
246 	different location, the manager will receive
247 	``UFFD_EVENT_REMAP``. The ``uffd_msg.remap`` will contain the old and
248 	new addresses of the area and its original length.
252 	madvise(MADV_DONTNEED) calls. The event ``UFFD_EVENT_REMOVE`` will
253 	be generated upon these calls to madvise(). The ``uffd_msg.remove``
254 	will contain start and end addresses of the removed area.
257 	enable notifications about memory unmapping. The manager will
259 	end addresses of the unmapped area.
261 Although the ``UFFD_FEATURE_EVENT_REMOVE`` and ``UFFD_FEATURE_EVENT_UNMAP``
262 are pretty similar, they quite differ in the action expected from the
263 ``userfaultfd`` manager. In the former case, the virtual memory is
264 removed, but the area is not, the area remains monitored by the
266 delivered to the manager. The proper resolution for such page fault is
267 to zeromap the faulting address. However, in the latter case, when an
269 implicitly (e.g. during mremap()), the area is removed and in turn the
270 ``userfaultfd`` context for such area disappears too and the manager will
271 not get further userland page faults from the removed area. Still, the
273 ``UFFDIO_COPY`` on the unmapped area.
276 explicit or implicit wakeup, all the events are delivered
277 asynchronously and the non-cooperative process resumes execution as
278 soon as manager executes read(). The ``userfaultfd`` manager should
279 carefully synchronize calls to ``UFFDIO_COPY`` with the events
280 processing. To aid the synchronization, the ``UFFDIO_COPY`` ioctl will
281 return ``-ENOSPC`` when the monitored process exits at the time of
282 ``UFFDIO_COPY``, and ``-ENOENT``, when the non-cooperative process has changed
286 The current asynchronous model of the event delivery is optimal for
289 ``userfaultfd`` feature to facilitate multithreading enhancements of the
291 run in parallel to the event reception. Single threaded
292 implementations should continue to use the current async event