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1 .\" Copyright (c) 2016, IBM Corporation.
2 .\" Written by Mike Rapoport <rppt@linux.vnet.ibm.com>
3 .\" and Copyright (C) 2016 Michael Kerrisk <mtk.manpages@gmail.com>
4 .\"
5 .\" SPDX-License-Identifier: Linux-man-pages-copyleft
6 .\"
7 .\"
8 .TH IOCTL_USERFAULTFD 2 2021-03-22 "Linux man-pages (unreleased)"
9 .SH NAME
10 ioctl_userfaultfd \- create a file descriptor for handling page faults in user
11 space
12 .SH LIBRARY
13 Standard C library
14 .RI ( libc ", " \-lc )
15 .SH SYNOPSIS
16 .nf
17 .BR "#include <linux/userfaultfd.h>" " /* Definition of " UFFD* " constants */"
18 .B #include <sys/ioctl.h>
19 .PP
20 .BI "int ioctl(int " fd ", int " cmd ", ...);"
21 .fi
22 .SH DESCRIPTION
23 Various
24 .BR ioctl (2)
25 operations can be performed on a userfaultfd object (created by a call to
26 .BR userfaultfd (2))
27 using calls of the form:
28 .PP
29 .in +4n
30 .EX
31 ioctl(fd, cmd, argp);
32 .EE
33 .in
34 In the above,
35 .I fd
36 is a file descriptor referring to a userfaultfd object,
37 .I cmd
38 is one of the commands listed below, and
39 .I argp
40 is a pointer to a data structure that is specific to
41 .IR cmd .
42 .PP
43 The various
44 .BR ioctl (2)
45 operations are described below.
46 The
47 .BR UFFDIO_API ,
48 .BR UFFDIO_REGISTER ,
49 and
50 .B UFFDIO_UNREGISTER
51 operations are used to
52 .I configure
53 userfaultfd behavior.
54 These operations allow the caller to choose what features will be enabled and
55 what kinds of events will be delivered to the application.
56 The remaining operations are
57 .I range
58 operations.
59 These operations enable the calling application to resolve page-fault
60 events.
61 .\"
62 .SS UFFDIO_API
63 (Since Linux 4.3.)
64 Enable operation of the userfaultfd and perform API handshake.
65 .PP
66 The
67 .I argp
68 argument is a pointer to a
69 .I uffdio_api
70 structure, defined as:
71 .PP
72 .in +4n
73 .EX
74 struct uffdio_api {
75 uint64_t api; /* Requested API version (input) */
76 uint64_t features; /* Requested features (input/output) */
77 uint64_t ioctls; /* Available ioctl() operations (output) */
78 };
79 .EE
80 .in
81 .PP
82 The
83 .I api
84 field denotes the API version requested by the application.
85 .PP
86 The kernel verifies that it can support the requested API version,
87 and sets the
88 .I features
89 and
90 .I ioctls
91 fields to bit masks representing all the available features and the generic
92 .BR ioctl (2)
93 operations available.
94 .PP
95 For Linux kernel versions before 4.11, the
96 .I features
97 field must be initialized to zero before the call to
98 .BR UFFDIO_API ,
99 and zero (i.e., no feature bits) is placed in the
100 .I features
101 field by the kernel upon return from
102 .BR ioctl (2).
103 .PP
104 Starting from Linux 4.11, the
105 .I features
106 field can be used to ask whether particular features are supported
107 and explicitly enable userfaultfd features that are disabled by default.
108 The kernel always reports all the available features in the
109 .I features
110 field.
111 .PP
112 To enable userfaultfd features the application should set
113 a bit corresponding to each feature it wants to enable in the
114 .I features
115 field.
116 If the kernel supports all the requested features it will enable them.
117 Otherwise it will zero out the returned
118 .I uffdio_api
119 structure and return
120 .BR EINVAL .
121 .\" FIXME add more details about feature negotiation and enablement
122 .PP
123 The following feature bits may be set:
124 .TP
125 .BR UFFD_FEATURE_EVENT_FORK " (since Linux 4.11)"
126 When this feature is enabled,
127 the userfaultfd objects associated with a parent process are duplicated
128 into the child process during
129 .BR fork (2)
130 and a
131 .B UFFD_EVENT_FORK
132 event is delivered to the userfaultfd monitor
133 .TP
134 .BR UFFD_FEATURE_EVENT_REMAP " (since Linux 4.11)"
135 If this feature is enabled,
136 when the faulting process invokes
137 .BR mremap (2),
138 the userfaultfd monitor will receive an event of type
139 .BR UFFD_EVENT_REMAP .
140 .TP
141 .BR UFFD_FEATURE_EVENT_REMOVE " (since Linux 4.11)"
142 If this feature is enabled,
143 when the faulting process calls
144 .BR madvise (2)
145 with the
146 .B MADV_DONTNEED
147 or
148 .B MADV_REMOVE
149 advice value to free a virtual memory area
150 the userfaultfd monitor will receive an event of type
151 .BR UFFD_EVENT_REMOVE .
152 .TP
153 .BR UFFD_FEATURE_EVENT_UNMAP " (since Linux 4.11)"
154 If this feature is enabled,
155 when the faulting process unmaps virtual memory either explicitly with
156 .BR munmap (2),
157 or implicitly during either
158 .BR mmap (2)
159 or
160 .BR mremap (2),
161 the userfaultfd monitor will receive an event of type
162 .BR UFFD_EVENT_UNMAP .
163 .TP
164 .BR UFFD_FEATURE_MISSING_HUGETLBFS " (since Linux 4.11)"
165 If this feature bit is set,
166 the kernel supports registering userfaultfd ranges on hugetlbfs
167 virtual memory areas
168 .TP
169 .BR UFFD_FEATURE_MISSING_SHMEM " (since Linux 4.11)"
170 If this feature bit is set,
171 the kernel supports registering userfaultfd ranges on shared memory areas.
172 This includes all kernel shared memory APIs:
173 System V shared memory,
174 .BR tmpfs (5),
175 shared mappings of
176 .IR /dev/zero ,
177 .BR mmap (2)
178 with the
179 .B MAP_SHARED
180 flag set,
181 .BR memfd_create (2),
182 and so on.
183 .TP
184 .BR UFFD_FEATURE_SIGBUS " (since Linux 4.14)"
185 .\" commit 2d6d6f5a09a96cc1fec7ed992b825e05f64cb50e
186 If this feature bit is set, no page-fault events
187 .RB ( UFFD_EVENT_PAGEFAULT )
188 will be delivered.
189 Instead, a
190 .B SIGBUS
191 signal will be sent to the faulting process.
192 Applications using this
193 feature will not require the use of a userfaultfd monitor for processing
194 memory accesses to the regions registered with userfaultfd.
195 .TP
196 .BR UFFD_FEATURE_THREAD_ID " (since Linux 4.14)"
197 If this feature bit is set,
198 .I uffd_msg.pagefault.feat.ptid
199 will be set to the faulted thread ID for each page-fault message.
200 .TP
201 .BR UFFD_FEATURE_MINOR_HUGETLBFS " (since Linux 5.13)"
202 If this feature bit is set,
203 the kernel supports registering userfaultfd ranges
204 in minor mode on hugetlbfs-backed memory areas.
205 .TP
206 .BR UFFD_FEATURE_MINOR_SHMEM " (since Linux 5.14)"
207 If this feature bit is set,
208 the kernel supports registering userfaultfd ranges
209 in minor mode on shmem-backed memory areas.
210 .PP
211 The returned
212 .I ioctls
213 field can contain the following bits:
214 .\" FIXME This user-space API seems not fully polished. Why are there
215 .\" not constants defined for each of the bit-mask values listed below?
216 .TP
217 .B 1 << _UFFDIO_API
218 The
219 .B UFFDIO_API
220 operation is supported.
221 .TP
222 .B 1 << _UFFDIO_REGISTER
223 The
224 .B UFFDIO_REGISTER
225 operation is supported.
226 .TP
227 .B 1 << _UFFDIO_UNREGISTER
228 The
229 .B UFFDIO_UNREGISTER
230 operation is supported.
231 .PP
232 This
233 .BR ioctl (2)
234 operation returns 0 on success.
235 On error, \-1 is returned and
236 .I errno
237 is set to indicate the error.
238 Possible errors include:
239 .TP
240 .B EFAULT
241 .I argp
242 refers to an address that is outside the calling process's
243 accessible address space.
244 .TP
245 .B EINVAL
246 The userfaultfd has already been enabled by a previous
247 .B UFFDIO_API
248 operation.
249 .TP
250 .B EINVAL
251 The API version requested in the
252 .I api
253 field is not supported by this kernel, or the
254 .I features
255 field passed to the kernel includes feature bits that are not supported
256 by the current kernel version.
257 .\" FIXME In the above error case, the returned 'uffdio_api' structure is
258 .\" zeroed out. Why is this done? This should be explained in the manual page.
259 .\"
260 .\" Mike Rapoport:
261 .\" In my understanding the uffdio_api
262 .\" structure is zeroed to allow the caller
263 .\" to distinguish the reasons for -EINVAL.
264 .\"
265 .SS UFFDIO_REGISTER
266 (Since Linux 4.3.)
267 Register a memory address range with the userfaultfd object.
268 The pages in the range must be "compatible".
269 Please refer to the list of register modes below
270 for the compatible memory backends for each mode.
271 .PP
272 The
273 .I argp
274 argument is a pointer to a
275 .I uffdio_register
276 structure, defined as:
277 .PP
278 .in +4n
279 .EX
280 struct uffdio_range {
281 uint64_t start; /* Start of range */
282 uint64_t len; /* Length of range (bytes) */
283 };
285 struct uffdio_register {
286 struct uffdio_range range;
287 uint64_t mode; /* Desired mode of operation (input) */
288 uint64_t ioctls; /* Available ioctl() operations (output) */
289 };
290 .EE
291 .in
292 .PP
293 The
294 .I range
295 field defines a memory range starting at
296 .I start
297 and continuing for
298 .I len
299 bytes that should be handled by the userfaultfd.
300 .PP
301 The
302 .I mode
303 field defines the mode of operation desired for this memory region.
304 The following values may be bitwise ORed to set the userfaultfd mode for
305 the specified range:
306 .TP
307 .B UFFDIO_REGISTER_MODE_MISSING
308 Track page faults on missing pages.
309 Since Linux 4.3,
310 only private anonymous ranges are compatible.
311 Since Linux 4.11,
312 hugetlbfs and shared memory ranges are also compatible.
313 .TP
314 .B UFFDIO_REGISTER_MODE_WP
315 Track page faults on write-protected pages.
316 Since Linux 5.7,
317 only private anonymous ranges are compatible.
318 .TP
319 .B UFFDIO_REGISTER_MODE_MINOR
320 Track minor page faults.
321 Since Linux 5.13,
322 only hugetlbfs ranges are compatible.
323 Since Linux 5.14,
324 compatiblity with shmem ranges was added.
325 .PP
326 If the operation is successful, the kernel modifies the
327 .I ioctls
328 bit-mask field to indicate which
329 .BR ioctl (2)
330 operations are available for the specified range.
331 This returned bit mask can contain the following bits:
332 .TP
333 .B 1 << _UFFDIO_COPY
334 The
335 .B UFFDIO_COPY
336 operation is supported.
337 .TP
338 .B 1 << _UFFDIO_WAKE
339 The
340 .B UFFDIO_WAKE
341 operation is supported.
342 .TP
343 .B 1 << _UFFDIO_WRITEPROTECT
344 The
345 .B UFFDIO_WRITEPROTECT
346 .TP
347 .B 1 << _UFFDIO_ZEROPAGE
348 The
349 .B UFFDIO_ZEROPAGE
350 operation is supported.
351 .TP
352 .B 1 << _UFFDIO_CONTINUE
353 The
354 .B UFFDIO_CONTINUE
355 operation is supported.
356 .PP
357 This
358 .BR ioctl (2)
359 operation returns 0 on success.
360 On error, \-1 is returned and
361 .I errno
362 is set to indicate the error.
363 Possible errors include:
364 .\" FIXME Is the following error list correct?
365 .\"
366 .TP
367 .B EBUSY
368 A mapping in the specified range is registered with another
369 userfaultfd object.
370 .TP
371 .B EFAULT
372 .I argp
373 refers to an address that is outside the calling process's
374 accessible address space.
375 .TP
376 .B EINVAL
377 An invalid or unsupported bit was specified in the
378 .I mode
379 field; or the
380 .I mode
381 field was zero.
382 .TP
383 .B EINVAL
384 There is no mapping in the specified address range.
385 .TP
386 .B EINVAL
387 .I range.start
388 or
389 .I range.len
390 is not a multiple of the system page size; or,
391 .I range.len
392 is zero; or these fields are otherwise invalid.
393 .TP
394 .B EINVAL
395 There as an incompatible mapping in the specified address range.
396 .\" Mike Rapoport:
397 .\" ENOMEM if the process is exiting and the
398 .\" mm_struct has gone by the time userfault grabs it.
399 .SS UFFDIO_UNREGISTER
400 (Since Linux 4.3.)
401 Unregister a memory address range from userfaultfd.
402 The pages in the range must be "compatible" (see the description of
403 .BR UFFDIO_REGISTER .)
404 .PP
405 The address range to unregister is specified in the
406 .I uffdio_range
407 structure pointed to by
408 .IR argp .
409 .PP
410 This
411 .BR ioctl (2)
412 operation returns 0 on success.
413 On error, \-1 is returned and
414 .I errno
415 is set to indicate the error.
416 Possible errors include:
417 .TP
418 .B EINVAL
419 Either the
420 .I start
421 or the
422 .I len
423 field of the
424 .I ufdio_range
425 structure was not a multiple of the system page size; or the
426 .I len
427 field was zero; or these fields were otherwise invalid.
428 .TP
429 .B EINVAL
430 There as an incompatible mapping in the specified address range.
431 .TP
432 .B EINVAL
433 There was no mapping in the specified address range.
434 .\"
435 .SS UFFDIO_COPY
436 (Since Linux 4.3.)
437 Atomically copy a continuous memory chunk into the userfault registered
438 range and optionally wake up the blocked thread.
439 The source and destination addresses and the number of bytes to copy are
440 specified by the
441 .IR src ", " dst ", and " len
442 fields of the
443 .I uffdio_copy
444 structure pointed to by
445 .IR argp :
446 .PP
447 .in +4n
448 .EX
449 struct uffdio_copy {
450 uint64_t dst; /* Destination of copy */
451 uint64_t src; /* Source of copy */
452 uint64_t len; /* Number of bytes to copy */
453 uint64_t mode; /* Flags controlling behavior of copy */
454 int64_t copy; /* Number of bytes copied, or negated error */
455 };
456 .EE
457 .in
458 .PP
459 The following value may be bitwise ORed in
460 .I mode
461 to change the behavior of the
462 .B UFFDIO_COPY
463 operation:
464 .TP
465 .B UFFDIO_COPY_MODE_DONTWAKE
466 Do not wake up the thread that waits for page-fault resolution
467 .TP
468 .B UFFDIO_COPY_MODE_WP
469 Copy the page with read-only permission.
470 This allows the user to trap the next write to the page,
471 which will block and generate another write-protect userfault message.
472 This is used only when both
473 .B UFFDIO_REGISTER_MODE_MISSING
474 and
475 .B UFFDIO_REGISTER_MODE_WP
476 modes are enabled for the registered range.
477 .PP
478 The
479 .I copy
480 field is used by the kernel to return the number of bytes
481 that was actually copied, or an error (a negated
482 .IR errno -style
483 value).
484 .\" FIXME Above: Why is the 'copy' field used to return error values?
485 .\" This should be explained in the manual page.
486 If the value returned in
487 .I copy
488 doesn't match the value that was specified in
489 .IR len ,
490 the operation fails with the error
491 .BR EAGAIN .
492 The
493 .I copy
494 field is output-only;
495 it is not read by the
496 .B UFFDIO_COPY
497 operation.
498 .PP
499 This
500 .BR ioctl (2)
501 operation returns 0 on success.
502 In this case, the entire area was copied.
503 On error, \-1 is returned and
504 .I errno
505 is set to indicate the error.
506 Possible errors include:
507 .TP
508 .B EAGAIN
509 The number of bytes copied (i.e., the value returned in the
510 .I copy
511 field)
512 does not equal the value that was specified in the
513 .I len
514 field.
515 .TP
516 .B EINVAL
517 Either
518 .I dst
519 or
520 .I len
521 was not a multiple of the system page size, or the range specified by
522 .I src
523 and
524 .I len
525 or
526 .I dst
527 and
528 .I len
529 was invalid.
530 .TP
531 .B EINVAL
532 An invalid bit was specified in the
533 .I mode
534 field.
535 .TP
536 .BR ENOENT " (since Linux 4.11)"
537 The faulting process has changed
538 its virtual memory layout simultaneously with an outstanding
539 .B UFFDIO_COPY
540 operation.
541 .TP
542 .BR ENOSPC " (from Linux 4.11 until Linux 4.13)"
543 The faulting process has exited at the time of a
544 .B UFFDIO_COPY
545 operation.
546 .TP
547 .BR ESRCH " (since Linux 4.13)"
548 The faulting process has exited at the time of a
549 .B UFFDIO_COPY
550 operation.
551 .\"
552 .SS UFFDIO_ZEROPAGE
553 (Since Linux 4.3.)
554 Zero out a memory range registered with userfaultfd.
555 .PP
556 The requested range is specified by the
557 .I range
558 field of the
559 .I uffdio_zeropage
560 structure pointed to by
561 .IR argp :
562 .PP
563 .in +4n
564 .EX
565 struct uffdio_zeropage {
566 struct uffdio_range range;
567 uint64_t mode; /* Flags controlling behavior of copy */
568 int64_t zeropage; /* Number of bytes zeroed, or negated error */
569 };
570 .EE
571 .in
572 .PP
573 The following value may be bitwise ORed in
574 .I mode
575 to change the behavior of the
576 .B UFFDIO_ZEROPAGE
577 operation:
578 .TP
579 .B UFFDIO_ZEROPAGE_MODE_DONTWAKE
580 Do not wake up the thread that waits for page-fault resolution.
581 .PP
582 The
583 .I zeropage
584 field is used by the kernel to return the number of bytes
585 that was actually zeroed,
586 or an error in the same manner as
587 .BR UFFDIO_COPY .
588 .\" FIXME Why is the 'zeropage' field used to return error values?
589 .\" This should be explained in the manual page.
590 If the value returned in the
591 .I zeropage
592 field doesn't match the value that was specified in
593 .IR range.len ,
594 the operation fails with the error
595 .BR EAGAIN .
596 The
597 .I zeropage
598 field is output-only;
599 it is not read by the
600 .B UFFDIO_ZEROPAGE
601 operation.
602 .PP
603 This
604 .BR ioctl (2)
605 operation returns 0 on success.
606 In this case, the entire area was zeroed.
607 On error, \-1 is returned and
608 .I errno
609 is set to indicate the error.
610 Possible errors include:
611 .TP
612 .B EAGAIN
613 The number of bytes zeroed (i.e., the value returned in the
614 .I zeropage
615 field)
616 does not equal the value that was specified in the
617 .I range.len
618 field.
619 .TP
620 .B EINVAL
621 Either
622 .I range.start
623 or
624 .I range.len
625 was not a multiple of the system page size; or
626 .I range.len
627 was zero; or the range specified was invalid.
628 .TP
629 .B EINVAL
630 An invalid bit was specified in the
631 .I mode
632 field.
633 .TP
634 .BR ESRCH " (since Linux 4.13)"
635 The faulting process has exited at the time of a
636 .B UFFDIO_ZEROPAGE
637 operation.
638 .\"
639 .SS UFFDIO_WAKE
640 (Since Linux 4.3.)
641 Wake up the thread waiting for page-fault resolution on
642 a specified memory address range.
643 .PP
644 The
645 .B UFFDIO_WAKE
646 operation is used in conjunction with
647 .B UFFDIO_COPY
648 and
649 .B UFFDIO_ZEROPAGE
650 operations that have the
651 .B UFFDIO_COPY_MODE_DONTWAKE
652 or
653 .B UFFDIO_ZEROPAGE_MODE_DONTWAKE
654 bit set in the
655 .I mode
656 field.
657 The userfault monitor can perform several
658 .B UFFDIO_COPY
659 and
660 .B UFFDIO_ZEROPAGE
661 operations in a batch and then explicitly wake up the faulting thread using
662 .BR UFFDIO_WAKE .
663 .PP
664 The
665 .I argp
666 argument is a pointer to a
667 .I uffdio_range
668 structure (shown above) that specifies the address range.
669 .PP
670 This
671 .BR ioctl (2)
672 operation returns 0 on success.
673 On error, \-1 is returned and
674 .I errno
675 is set to indicate the error.
676 Possible errors include:
677 .TP
678 .B EINVAL
679 The
680 .I start
681 or the
682 .I len
683 field of the
684 .I ufdio_range
685 structure was not a multiple of the system page size; or
686 .I len
687 was zero; or the specified range was otherwise invalid.
688 .SS UFFDIO_WRITEPROTECT (Since Linux 5.7)
689 Write-protect or write-unprotect a userfaultfd-registered memory range
690 registered with mode
691 .BR UFFDIO_REGISTER_MODE_WP .
692 .PP
693 The
694 .I argp
695 argument is a pointer to a
696 .I uffdio_range
697 structure as shown below:
698 .PP
699 .in +4n
700 .EX
701 struct uffdio_writeprotect {
702 struct uffdio_range range; /* Range to change write permission*/
703 uint64_t mode; /* Mode to change write permission */
704 };
705 .EE
706 .in
707 .PP
708 There are two mode bits that are supported in this structure:
709 .TP
710 .B UFFDIO_WRITEPROTECT_MODE_WP
711 When this mode bit is set,
712 the ioctl will be a write-protect operation upon the memory range specified by
713 .IR range .
714 Otherwise it will be a write-unprotect operation upon the specified range,
715 which can be used to resolve a userfaultfd write-protect page fault.
716 .TP
717 .B UFFDIO_WRITEPROTECT_MODE_DONTWAKE
718 When this mode bit is set,
719 do not wake up any thread that waits for
720 page-fault resolution after the operation.
721 This can be specified only if
722 .B UFFDIO_WRITEPROTECT_MODE_WP
723 is not specified.
724 .PP
725 This
726 .BR ioctl (2)
727 operation returns 0 on success.
728 On error, \-1 is returned and
729 .I errno
730 is set to indicate the error.
731 Possible errors include:
732 .TP
733 .B EINVAL
734 The
735 .I start
736 or the
737 .I len
738 field of the
739 .I ufdio_range
740 structure was not a multiple of the system page size; or
741 .I len
742 was zero; or the specified range was otherwise invalid.
743 .TP
744 .B EAGAIN
745 The process was interrupted; retry this call.
746 .TP
747 .B ENOENT
748 The range specified in
749 .I range
750 is not valid.
751 For example, the virtual address does not exist,
752 or not registered with userfaultfd write-protect mode.
753 .TP
754 .B EFAULT
755 Encountered a generic fault during processing.
756 .\"
757 .SS UFFDIO_CONTINUE
758 (Since Linux 5.13.)
759 Resolve a minor page fault
760 by installing page table entries
761 for existing pages in the page cache.
762 .PP
763 The
764 .I argp
765 argument is a pointer to a
766 .I uffdio_continue
767 structure as shown below:
768 .PP
769 .in +4n
770 .EX
771 struct uffdio_continue {
772 struct uffdio_range range;
773 /* Range to install PTEs for and continue */
774 uint64_t mode; /* Flags controlling the behavior of continue */
775 int64_t mapped; /* Number of bytes mapped, or negated error */
776 };
777 .EE
778 .in
779 .PP
780 The following value may be bitwise ORed in
781 .I mode
782 to change the behavior of the
783 .B UFFDIO_CONTINUE
784 operation:
785 .TP
786 .B UFFDIO_CONTINUE_MODE_DONTWAKE
787 Do not wake up the thread that waits for page-fault resolution.
788 .PP
789 The
790 .I mapped
791 field is used by the kernel
792 to return the number of bytes that were actually mapped,
793 or an error in the same manner as
794 .BR UFFDIO_COPY .
795 If the value returned in the
796 .I mapped
797 field doesn't match the value that was specified in
798 .IR range.len ,
799 the operation fails with the error
800 .BR EAGAIN .
801 The
802 .I mapped
803 field is output-only;
804 it is not read by the
805 .B UFFDIO_CONTINUE
806 operation.
807 .PP
808 This
809 .BR ioctl (2)
810 operation returns 0 on success.
811 In this case,
812 the entire area was mapped.
813 On error, \-1 is returned and
814 .I errno
815 is set to indicate the error.
816 Possible errors include:
817 .TP
818 .B EAGAIN
819 The number of bytes mapped
820 (i.e., the value returned in the
821 .I mapped
822 field)
823 does not equal the value that was specified in the
824 .I range.len
825 field.
826 .TP
827 .B EINVAL
828 Either
829 .I range.start
830 or
831 .I range.len
832 was not a multiple of the system page size; or
833 .I range.len
834 was zero; or the range specified was invalid.
835 .TP
836 .B EINVAL
837 An invalid bit was specified in the
838 .I mode
839 field.
840 .TP
841 .B EEXIST
842 One or more pages were already mapped in the given range.
843 .TP
844 .B ENOENT
845 The faulting process has changed its virtual memory layout simultaneously with
846 an outstanding
847 .B UFFDIO_CONTINUE
848 operation.
849 .TP
850 .B ENOMEM
851 Allocating memory needed to setup the page table mappings failed.
852 .TP
853 .B EFAULT
854 No existing page could be found in the page cache for the given range.
855 .TP
856 .B ESRCH
857 The faulting process has exited at the time of a
858 .B UFFDIO_CONTINUE
859 operation.
860 .\"
861 .SH RETURN VALUE
862 See descriptions of the individual operations, above.
863 .SH ERRORS
864 See descriptions of the individual operations, above.
865 In addition, the following general errors can occur for all of the
866 operations described above:
867 .TP
868 .B EFAULT
869 .I argp
870 does not point to a valid memory address.
871 .TP
872 .B EINVAL
873 (For all operations except
874 .BR UFFDIO_API .)
875 The userfaultfd object has not yet been enabled (via the
876 .B UFFDIO_API
877 operation).
878 .SH STANDARDS
879 These
880 .BR ioctl (2)
881 operations are Linux-specific.
882 .SH BUGS
883 In order to detect available userfault features and
884 enable some subset of those features
885 the userfaultfd file descriptor must be closed after the first
886 .B UFFDIO_API
887 operation that queries features availability and reopened before
888 the second
889 .B UFFDIO_API
890 operation that actually enables the desired features.
891 .SH EXAMPLES
892 See
893 .BR userfaultfd (2).
894 .SH SEE ALSO
895 .BR ioctl (2),
896 .BR mmap (2),
897 .BR userfaultfd (2)
898 .PP
899 .I Documentation/admin\-guide/mm/userfaultfd.rst
900 in the Linux kernel source tree