Lines Matching full:the
8 Userfaults allow the implementation of on-demand paging from userland
10 memory page faults, something otherwise only the kernel code could do.
13 of the ``PROT_NONE+SIGSEGV`` trick.
19 regions of virtual memory with it. Then, any page faults which occur within the
20 region(s) result in a message being delivered to the userfaultfd, notifying
21 userspace of the fault.
23 The ``userfaultfd`` (aside from registering and unregistering virtual
26 1) ``read/POLLIN`` protocol to notify a userland thread of the faults
29 2) various ``UFFDIO_*`` ioctls that can manage the virtual memory regions
30 registered in the ``userfaultfd`` that allows userland to efficiently
31 resolve the userfaults it receives via 1) or to manage the virtual
32 memory in the background
34 The real advantage of userfaults if compared to regular virtual memory
35 management of mremap/mprotect is that the userfaults in all their
36 operations never involve heavyweight structures like vmas (in fact the
37 ``userfaultfd`` runtime load never takes the mmap_lock for writing).
42 The ``userfaultfd``, once created, 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
58 handle kernel page faults have been a useful tool for exploiting the kernel).
60 The first way, supported since userfaultfd was introduced, is the
64 only. Such a userfaultfd can be created using the userfaultfd(2) syscall
65 with the flag UFFD_USER_MODE_ONLY.
67 - In order to also trap kernel page faults for the address space, either the
68 process needs the CAP_SYS_PTRACE capability, or the system must have
72 The second way, added to the kernel more recently, is by opening
74 yields equivalent userfaultfds to the userfaultfd(2) syscall.
79 the same time (as e.g. granting CAP_SYS_PTRACE would do). Users who have access
86 When first opened the ``userfaultfd`` must be enabled invoking the
88 a later API version) which will specify the ``read/POLLIN`` protocol
89 userland intends to speak on the ``UFFD`` and the ``uffdio_api.features``
90 userland requires. The ``UFFDIO_API`` ioctl if successful (i.e. if the
91 requested ``uffdio_api.api`` is spoken also by the running kernel and the
94 respectively all the available features of the read(2) protocol and
95 the generic ioctl available.
97 The ``uffdio_api.features`` bitmask returned by the ``UFFDIO_API`` ioctl
98 defines what memory types are supported by the ``userfaultfd`` and what
101 - The ``UFFD_FEATURE_EVENT_*`` flags indicate that various other events
103 detail below in the `Non-cooperative userfaultfd`_ section.
106 indicate that the kernel supports ``UFFDIO_REGISTER_MODE_MISSING``
111 - ``UFFD_FEATURE_MINOR_HUGETLBFS`` indicates that the kernel supports
113 areas. ``UFFD_FEATURE_MINOR_SHMEM`` is the analogous feature indicating
116 - ``UFFD_FEATURE_MOVE`` indicates that the kernel supports moving an
119 The userland application should set the feature flags it intends to use
120 when invoking the ``UFFDIO_API`` ioctl, to request that those features be
123 Once the ``userfaultfd`` API has been enabled the ``UFFDIO_REGISTER``
124 ioctl should be invoked (if present in the returned ``uffdio_api.ioctls``
125 bitmask) to register a memory range in the ``userfaultfd`` by setting the
126 uffdio_register structure accordingly. The ``uffdio_register.mode``
127 bitmask will specify to the kernel which kind of faults to track for
128 the range. The ``UFFDIO_REGISTER`` ioctl will return the
130 userfaults on the range registered. Not all ioctls will necessarily be
134 Userland can use the ``uffdio_register.ioctls`` to manage the virtual
135 address space in the background (to add or potentially also remove
136 memory from the ``userfaultfd`` registered range). This means a userfault
137 could be triggering just before userland maps in the background the
148 - ``UFFDIO_ZEROPAGE`` atomically zeros the new page.
152 These operations are atomic in the sense that they guarantee nothing can
153 see a half-populated page, since readers will keep userfaulting until the
156 By default, these wake up userfaults blocked on the range in question.
160 Which ioctl to choose depends on the kind of page fault, and what we'd
163 - For ``UFFDIO_REGISTER_MODE_MISSING`` faults, the fault needs to be
165 the zero page (``UFFDIO_ZEROPAGE``). By default, the kernel would map
166 the zero page for a missing fault. With userfaultfd, userspace can
167 decide what content to provide before the faulting thread continues.
170 the page cache). Userspace has the option of modifying the page's
171 contents before resolving the fault. Once the contents are correct
172 (modified or not), userspace asks the kernel to map the page and let the
178 ``pagefault.flags`` within the ``uffd_msg``, checking for the
181 - None of the page-delivering ioctls default to the range that you
182 registered with. You must fill in all fields for the appropriate
183 ioctl struct including the range.
185 - You get the address of the access that triggered the missing page
186 event out of a struct uffd_msg that you read in the thread from the
188 Keep in mind that unless you used DONTWAKE then the first of any of
189 those IOCTLs wakes up the faulting thread.
205 in the struct passed in. The range does not default to and does not
206 have to be identical to the range you registered with. You can write
207 protect as many ranges as you like (inside the registered range).
208 Then, in the thread reading from uffd the struct will have
212 set. This wakes up the thread which will continue to run with writes. This
213 allows you to do the bookkeeping about the write in the uffd reading
214 thread before the ioctl.
217 ``UFFDIO_REGISTER_MODE_WP`` then you need to think about the sequence in
219 difference between writes into a WP area and into a !WP area. The
220 former will have ``UFFD_PAGEFAULT_FLAG_WP`` set, the latter
221 ``UFFD_PAGEFAULT_FLAG_WRITE``. The latter did not fail on protection but
233 as long as the page range was write-protected before. Such a message will
236 If the application wants to be able to write protect none ptes on anonymous
237 memory, one can pre-populate the memory with e.g. MADV_POPULATE_READ. On
238 newer kernels, one can also detect the feature UFFD_FEATURE_WP_UNPOPULATED
239 and set the feature bit in advance to make sure none ptes will also be
245 respectively, it may be desirable for the new page / mapping to be
248 respectively) to configure the mapping this way.
250 If the userfaultfd context has ``UFFD_FEATURE_WP_ASYNC`` feature bit set,
252 than the default sync mode.
255 happens, meanwhile the write-protection will be resolved automatically by
256 the kernel. It can be seen as a more accurate version of soft-dirty
259 - The dirty result will not be affected by vma changes (e.g. vma
260 merging) because the dirty is only tracked by the pte.
265 - Dirty information will not get lost if the pte was zapped due to
270 some of the memory operations. For example: ``MADV_DONTNEED`` on
272 dirtying of memory by dropping uffd-wp bit during the procedure.
274 The user app can collect the "written/dirty" status by looking up the
275 uffd-wp bit for the pages being interested in /proc/pagemap.
277 The page will not be under track of uffd-wp async mode until the page is
278 explicitly write-protected by ``ioctl(UFFDIO_WRITEPROTECT)`` with the mode
290 future faulters to either get a SIGBUS, or in KVM's case the guest will
295 the VM to another physical machine. Since we want the migration to be
296 transparent to the guest, we want that same address range to act as if it was
298 doesn't have a memory error in the exact same spot.
303 QEMU/KVM is using the ``userfaultfd`` syscall to implement postcopy live
306 all of its memory residing on a different node in the cloud. The
313 page faults in the guest scheduler so those guest processes that
315 the guest vcpus.
321 The implementation of postcopy live migration currently uses one
322 single bidirectional socket but in the future two different sockets
323 will be used (to reduce the latency of the userfaults to the minimum
326 The QEMU in the source node writes all pages that it knows are missing
327 in the destination node, into the socket, and the migration thread of
328 the QEMU running in the destination node runs ``UFFDIO_COPY|ZEROPAGE``
329 ioctls on the ``userfaultfd`` in order to map the received pages into the
330 guest (``UFFDIO_ZEROCOPY`` is used if the source page was a zero page).
332 A different postcopy thread in the destination node listens with
333 poll() to the ``userfaultfd`` in parallel. When a ``POLLIN`` event is
334 generated after a userfault triggers, the postcopy thread read() from
335 the ``userfaultfd`` and receives the fault address (or ``-EAGAIN`` in case the
337 by the parallel QEMU migration thread).
339 After the QEMU postcopy thread (running in the destination node) gets
340 the userfault address it writes the information about the missing page
341 into the socket. The QEMU source node receives the information and
344 (just the time to flush the tcp_wmem queue through the network) the
345 migration thread in the QEMU running in the destination node will
346 receive the page that triggered the userfault and it'll map it as
347 usual with the ``UFFDIO_COPY|ZEROPAGE`` (without actually knowing if it
348 was spontaneously sent by the source or if it was an urgent page
351 By the time the userfaults start, the QEMU in the destination node
352 doesn't need to keep any per-page state bitmap relative to the live
354 the QEMU running in the source node to know which pages are still
355 missing in the destination node. The bitmap in the source node is
358 course the bitmap is updated accordingly. It's also useful to avoid
359 sending the same page twice (in case the userfault is read by the
360 postcopy thread just before ``UFFDIO_COPY|ZEROPAGE`` runs in the migration
366 When the ``userfaultfd`` is monitored by an external manager, the manager
367 must be able to track changes in the process virtual memory
368 layout. Userfaultfd can notify the manager about such changes using
369 the same read(2) protocol as for the page fault notifications. The
375 enabled, the ``userfaultfd`` context of the parent process is
376 duplicated into the newly created process. The manager
377 receives ``UFFD_EVENT_FORK`` with file descriptor of the new
378 ``userfaultfd`` context in the ``uffd_msg.fork``.
381 enable notifications about mremap() calls. When the
383 different location, the manager will receive
384 ``UFFD_EVENT_REMAP``. The ``uffd_msg.remap`` will contain the old and
385 new addresses of the area and its original length.
389 madvise(MADV_DONTNEED) calls. The event ``UFFD_EVENT_REMOVE`` will
390 be generated upon these calls to madvise(). The ``uffd_msg.remove``
391 will contain start and end addresses of the removed area.
394 enable notifications about memory unmapping. The manager will
396 end addresses of the unmapped area.
398 Although the ``UFFD_FEATURE_EVENT_REMOVE`` and ``UFFD_FEATURE_EVENT_UNMAP``
399 are pretty similar, they quite differ in the action expected from the
400 ``userfaultfd`` manager. In the former case, the virtual memory is
401 removed, but the area is not, the area remains monitored by the
403 delivered to the manager. The proper resolution for such page fault is
404 to zeromap the faulting address. However, in the latter case, when an
406 implicitly (e.g. during mremap()), the area is removed and in turn the
407 ``userfaultfd`` context for such area disappears too and the manager will
408 not get further userland page faults from the removed area. Still, the
410 ``UFFDIO_COPY`` on the unmapped area.
413 explicit or implicit wakeup, all the events are delivered
414 asynchronously and the non-cooperative process resumes execution as
415 soon as manager executes read(). The ``userfaultfd`` manager should
416 carefully synchronize calls to ``UFFDIO_COPY`` with the events
417 processing. To aid the synchronization, the ``UFFDIO_COPY`` ioctl will
418 return ``-ENOSPC`` when the monitored process exits at the time of
419 ``UFFDIO_COPY``, and ``-ENOENT``, when the non-cooperative process has changed
423 The current asynchronous model of the event delivery is optimal for
426 ``userfaultfd`` feature to facilitate multithreading enhancements of the
428 run in parallel to the event reception. Single threaded
429 implementations should continue to use the current async event