1USERFAULTFD(2) Linux Programmer's Manual USERFAULTFD(2)
2
6 userfaultfd - create a file descriptor for handling page faults in user
7 space
8
10 #include <sys/types.h>
11 #include <linux/userfaultfd.h>
12 int userfaultfd(int flags);
14 Note: There is no glibc wrapper for this system call; see NOTES.
16
18 userfaultfd() creates a new userfaultfd object that can be used for
19 delegation of page-fault handling to a user-space application, and re‐
20 turns a file descriptor that refers to the new object. The new user‐
21 faultfd object is configured using ioctl(2).
22 Once the userfaultfd object is configured, the application can use
24 read(2) to receive userfaultfd notifications. The reads from user‐
25 faultfd may be blocking or non-blocking, depending on the value of
26 flags used for the creation of the userfaultfd or subsequent calls to
27 fcntl(2).
28 The following values may be bitwise ORed in flags to change the behav‐
30 ior of userfaultfd():
31 O_CLOEXEC
33 Enable the close-on-exec flag for the new userfaultfd file de‐
34 scriptor. See the description of the O_CLOEXEC flag in open(2).
35 O_NONBLOCK
37 Enables non-blocking operation for the userfaultfd object. See
38 the description of the O_NONBLOCK flag in open(2).
39 When the last file descriptor referring to a userfaultfd object is
41 closed, all memory ranges that were registered with the object are un‐
42 registered and unread events are flushed.
43 Usage
45 The userfaultfd mechanism is designed to allow a thread in a multi‐
46 threaded program to perform user-space paging for the other threads in
47 the process. When a page fault occurs for one of the regions regis‐
48 tered to the userfaultfd object, the faulting thread is put to sleep
49 and an event is generated that can be read via the userfaultfd file de‐
50 scriptor. The fault-handling thread reads events from this file de‐
51 scriptor and services them using the operations described in
52 ioctl_userfaultfd(2). When servicing the page fault events, the fault-
53 handling thread can trigger a wake-up for the sleeping thread.
54 It is possible for the faulting threads and the fault-handling threads
56 to run in the context of different processes. In this case, these
57 threads may belong to different programs, and the program that executes
58 the faulting threads will not necessarily cooperate with the program
59 that handles the page faults. In such non-cooperative mode, the
60 process that monitors userfaultfd and handles page faults needs to be
61 aware of the changes in the virtual memory layout of the faulting
62 process to avoid memory corruption.
63 Starting from Linux 4.11, userfaultfd can also notify the fault-han‐
65 dling threads about changes in the virtual memory layout of the fault‐
66 ing process. In addition, if the faulting process invokes fork(2), the
67 userfaultfd objects associated with the parent may be duplicated into
68 the child process and the userfaultfd monitor will be notified (via the
69 UFFD_EVENT_FORK described below) about the file descriptor associated
70 with the userfault objects created for the child process, which allows
71 the userfaultfd monitor to perform user-space paging for the child
72 process. Unlike page faults which have to be synchronous and require
73 an explicit or implicit wakeup, all other events are delivered asyn‐
74 chronously and the non-cooperative process resumes execution as soon as
75 the userfaultfd manager executes read(2). The userfaultfd manager
76 should carefully synchronize calls to UFFDIO_COPY with the processing
77 of events.
78 The current asynchronous model of the event delivery is optimal for
80 single threaded non-cooperative userfaultfd manager implementations.
81 Userfaultfd operation
83 After the userfaultfd object is created with userfaultfd(), the appli‐
84 cation must enable it using the UFFDIO_API ioctl(2) operation. This
85 operation allows a handshake between the kernel and user space to de‐
86 termine the API version and supported features. This operation must be
87 performed before any of the other ioctl(2) operations described below
88 (or those operations fail with the EINVAL error).
89 After a successful UFFDIO_API operation, the application then registers
91 memory address ranges using the UFFDIO_REGISTER ioctl(2) operation.
92 After successful completion of a UFFDIO_REGISTER operation, a page
93 fault occurring in the requested memory range, and satisfying the mode
94 defined at the registration time, will be forwarded by the kernel to
95 the user-space application. The application can then use the UFF‐
96 DIO_COPY or UFFDIO_ZEROPAGE ioctl(2) operations to resolve the page
97 fault.
98 Starting from Linux 4.14, if the application sets the UFFD_FEATURE_SIG‐
100 BUS feature bit using the UFFDIO_API ioctl(2), no page-fault notifica‐
101 tion will be forwarded to user space. Instead a SIGBUS signal is de‐
102 livered to the faulting process. With this feature, userfaultfd can be
103 used for robustness purposes to simply catch any access to areas within
104 the registered address range that do not have pages allocated, without
105 having to listen to userfaultfd events. No userfaultfd monitor will be
106 required for dealing with such memory accesses. For example, this fea‐
107 ture can be useful for applications that want to prevent the kernel
108 from automatically allocating pages and filling holes in sparse files
109 when the hole is accessed through a memory mapping.
110 The UFFD_FEATURE_SIGBUS feature is implicitly inherited through fork(2)
112 if used in combination with UFFD_FEATURE_FORK.
113 Details of the various ioctl(2) operations can be found in ioctl_user‐
115 faultfd(2).
116 Since Linux 4.11, events other than page-fault may enabled during UFF‐
118 DIO_API operation.
119 Up to Linux 4.11, userfaultfd can be used only with anonymous private
121 memory mappings. Since Linux 4.11, userfaultfd can be also used with
122 hugetlbfs and shared memory mappings.
123 Reading from the userfaultfd structure
125 Each read(2) from the userfaultfd file descriptor returns one or more
126 uffd_msg structures, each of which describes a page-fault event or an
127 event required for the non-cooperative userfaultfd usage:
128 struct uffd_msg {
130 __u8 event; /* Type of event */
131 ...
132 union {
133 struct {
134 __u64 flags; /* Flags describing fault */
135 __u64 address; /* Faulting address */
136 } pagefault;
137 struct { /* Since Linux 4.11 */
139 __u32 ufd; /* Userfault file descriptor
140 of the child process */
141 } fork;
142 struct { /* Since Linux 4.11 */
144 __u64 from; /* Old address of remapped area */
145 __u64 to; /* New address of remapped area */
146 __u64 len; /* Original mapping length */
147 } remap;
148 struct { /* Since Linux 4.11 */
150 __u64 start; /* Start address of removed area */
151 __u64 end; /* End address of removed area */
152 } remove;
153 ...
154 } arg;
155 /* Padding fields omitted */
157 } __packed;
158 If multiple events are available and the supplied buffer is large
160 enough, read(2) returns as many events as will fit in the supplied buf‐
161 fer. If the buffer supplied to read(2) is smaller than the size of the
162 uffd_msg structure, the read(2) fails with the error EINVAL.
163 The fields set in the uffd_msg structure are as follows:
165 event The type of event. Depending of the event type, different
167 fields of the arg union represent details required for the event
168 processing. The non-page-fault events are generated only when
169 appropriate feature is enabled during API handshake with UFF‐
170 DIO_API ioctl(2).
171 The following values can appear in the event field:
173 UFFD_EVENT_PAGEFAULT (since Linux 4.3)
175 A page-fault event. The page-fault details are available
176 in the pagefault field.
177 UFFD_EVENT_FORK (since Linux 4.11)
179 Generated when the faulting process invokes fork(2) (or
180 clone(2) without the CLONE_VM flag). The event details
181 are available in the fork field.
182 UFFD_EVENT_REMAP (since Linux 4.11)
184 Generated when the faulting process invokes mremap(2).
185 The event details are available in the remap field.
186 UFFD_EVENT_REMOVE (since Linux 4.11)
188 Generated when the faulting process invokes madvise(2)
189 with MADV_DONTNEED or MADV_REMOVE advice. The event de‐
190 tails are available in the remove field.
191 UFFD_EVENT_UNMAP (since Linux 4.11)
193 Generated when the faulting process unmaps a memory
194 range, either explicitly using munmap(2) or implicitly
195 during mmap(2) or mremap(2). The event details are
196 available in the remove field.
197 pagefault.address
199 The address that triggered the page fault.
200 pagefault.flags
202 A bit mask of flags that describe the event. For
203 UFFD_EVENT_PAGEFAULT, the following flag may appear:
204 UFFD_PAGEFAULT_FLAG_WRITE
206 If the address is in a range that was registered with the
207 UFFDIO_REGISTER_MODE_MISSING flag (see ioctl_user‐
208 faultfd(2)) and this flag is set, this a write fault;
209 otherwise it is a read fault.
210 fork.ufd
212 The file descriptor associated with the userfault object created
213 for the child created by fork(2).
214 remap.from
216 The original address of the memory range that was remapped using
217 mremap(2).
218 remap.to
220 The new address of the memory range that was remapped using
221 mremap(2).
222 remap.len
224 The original length of the memory range that was remapped using
225 mremap(2).
226 remove.start
228 The start address of the memory range that was freed using mad‐
229 vise(2) or unmapped
230 remove.end
232 The end address of the memory range that was freed using mad‐
233 vise(2) or unmapped
234 A read(2) on a userfaultfd file descriptor can fail with the following
236 errors:
237 EINVAL The userfaultfd object has not yet been enabled using the UFF‐
239 DIO_API ioctl(2) operation
240 If the O_NONBLOCK flag is enabled in the associated open file descrip‐
242 tion, the userfaultfd file descriptor can be monitored with poll(2),
243 select(2), and epoll(7). When events are available, the file descrip‐
244 tor indicates as readable. If the O_NONBLOCK flag is not enabled, then
245 poll(2) (always) indicates the file as having a POLLERR condition, and
246 select(2) indicates the file descriptor as both readable and writable.
247
249 On success, userfaultfd() returns a new file descriptor that refers to
250 the userfaultfd object. On error, -1 is returned, and errno is set ap‐
251 propriately.
252
254 EINVAL An unsupported value was specified in flags.
255 EMFILE The per-process limit on the number of open file descriptors has
257 been reached
258 ENFILE The system-wide limit on the total number of open files has been
260 reached.
261 ENOMEM Insufficient kernel memory was available.
263 EPERM (since Linux 5.2)
265 The caller is not privileged (does not have the CAP_SYS_PTRACE
266 capability in the initial user namespace), and /proc/sys/vm/un‐
267 privileged_userfaultfd has the value 0.
268
270 The userfaultfd() system call first appeared in Linux 4.3.
271 The support for hugetlbfs and shared memory areas and non-page-fault
273 events was added in Linux 4.11
274
276 userfaultfd() is Linux-specific and should not be used in programs in‐
277 tended to be portable.
278
280 Glibc does not provide a wrapper for this system call; call it using
281 syscall(2).
282 The userfaultfd mechanism can be used as an alternative to traditional
284 user-space paging techniques based on the use of the SIGSEGV signal and
285 mmap(2). It can also be used to implement lazy restore for check‐
286 point/restore mechanisms, as well as post-copy migration to allow
287 (nearly) uninterrupted execution when transferring virtual machines and
288 Linux containers from one host to another.
289
291 If the UFFD_FEATURE_EVENT_FORK is enabled and a system call from the
292 fork(2) family is interrupted by a signal or failed, a stale user‐
293 faultfd descriptor might be created. In this case, a spurious
294 UFFD_EVENT_FORK will be delivered to the userfaultfd monitor.
295
297 The program below demonstrates the use of the userfaultfd mechanism.
298 The program creates two threads, one of which acts as the page-fault
299 handler for the process, for the pages in a demand-page zero region
300 created using mmap(2).
301 The program takes one command-line argument, which is the number of
303 pages that will be created in a mapping whose page faults will be han‐
304 dled via userfaultfd. After creating a userfaultfd object, the program
305 then creates an anonymous private mapping of the specified size and
306 registers the address range of that mapping using the UFFDIO_REGISTER
307 ioctl(2) operation. The program then creates a second thread that will
308 perform the task of handling page faults.
309 The main thread then walks through the pages of the mapping fetching
311 bytes from successive pages. Because the pages have not yet been ac‐
312 cessed, the first access of a byte in each page will trigger a page-
313 fault event on the userfaultfd file descriptor.
314 Each of the page-fault events is handled by the second thread, which
316 sits in a loop processing input from the userfaultfd file descriptor.
317 In each loop iteration, the second thread first calls poll(2) to check
318 the state of the file descriptor, and then reads an event from the file
319 descriptor. All such events should be UFFD_EVENT_PAGEFAULT events,
320 which the thread handles by copying a page of data into the faulting
321 region using the UFFDIO_COPY ioctl(2) operation.
322 The following is an example of what we see when running the program:
324 $ ./userfaultfd_demo 3
326 Address returned by mmap() = 0x7fd30106c000
327 fault_handler_thread():
329 poll() returns: nready = 1; POLLIN = 1; POLLERR = 0
330 UFFD_EVENT_PAGEFAULT event: flags = 0; address = 7fd30106c00f
331 (uffdio_copy.copy returned 4096)
332 Read address 0x7fd30106c00f in main(): A
333 Read address 0x7fd30106c40f in main(): A
334 Read address 0x7fd30106c80f in main(): A
335 Read address 0x7fd30106cc0f in main(): A
336 fault_handler_thread():
338 poll() returns: nready = 1; POLLIN = 1; POLLERR = 0
339 UFFD_EVENT_PAGEFAULT event: flags = 0; address = 7fd30106d00f
340 (uffdio_copy.copy returned 4096)
341 Read address 0x7fd30106d00f in main(): B
342 Read address 0x7fd30106d40f in main(): B
343 Read address 0x7fd30106d80f in main(): B
344 Read address 0x7fd30106dc0f in main(): B
345 fault_handler_thread():
347 poll() returns: nready = 1; POLLIN = 1; POLLERR = 0
348 UFFD_EVENT_PAGEFAULT event: flags = 0; address = 7fd30106e00f
349 (uffdio_copy.copy returned 4096)
350 Read address 0x7fd30106e00f in main(): C
351 Read address 0x7fd30106e40f in main(): C
352 Read address 0x7fd30106e80f in main(): C
353 Read address 0x7fd30106ec0f in main(): C
354 Program source
356 /* userfaultfd_demo.c
358 Licensed under the GNU General Public License version 2 or later.
360 */
361 #define _GNU_SOURCE
362 #include <inttypes.h>
363 #include <sys/types.h>
364 #include <stdio.h>
365 #include <linux/userfaultfd.h>
366 #include <pthread.h>
367 #include <errno.h>
368 #include <unistd.h>
369 #include <stdlib.h>
370 #include <fcntl.h>
371 #include <signal.h>
372 #include <poll.h>
373 #include <string.h>
374 #include <sys/mman.h>
375 #include <sys/syscall.h>
376 #include <sys/ioctl.h>
377 #include <poll.h>
378 #define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \
380 } while (0)
381 static int page_size;
383 static void *
385 fault_handler_thread(void *arg)
386 {
387 static struct uffd_msg msg; /* Data read from userfaultfd */
388 static int fault_cnt = 0; /* Number of faults so far handled */
389 long uffd; /* userfaultfd file descriptor */
390 static char *page = NULL;
391 struct uffdio_copy uffdio_copy;
392 ssize_t nread;
393 uffd = (long) arg;
395 /* Create a page that will be copied into the faulting region */
397 if (page == NULL) {
399 page = mmap(NULL, page_size, PROT_READ | PROT_WRITE,
400 MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
401 if (page == MAP_FAILED)
402 errExit("mmap");
403 }
404 /* Loop, handling incoming events on the userfaultfd
406 file descriptor */
407 for (;;) {
409 /* See what poll() tells us about the userfaultfd */
411 struct pollfd pollfd;
413 int nready;
414 pollfd.fd = uffd;
415 pollfd.events = POLLIN;
416 nready = poll(&pollfd, 1, -1);
417 if (nready == -1)
418 errExit("poll");
419 printf("\nfault_handler_thread():\n");
421 printf(" poll() returns: nready = %d; "
422 "POLLIN = %d; POLLERR = %d\n", nready,
423 (pollfd.revents & POLLIN) != 0,
424 (pollfd.revents & POLLERR) != 0);
425 /* Read an event from the userfaultfd */
427 nread = read(uffd, &msg, sizeof(msg));
429 if (nread == 0) {
430 printf("EOF on userfaultfd!\n");
431 exit(EXIT_FAILURE);
432 }
433 if (nread == -1)
435 errExit("read");
436 /* We expect only one kind of event; verify that assumption */
438 if (msg.event != UFFD_EVENT_PAGEFAULT) {
440 fprintf(stderr, "Unexpected event on userfaultfd\n");
441 exit(EXIT_FAILURE);
442 }
443 /* Display info about the page-fault event */
445 printf(" UFFD_EVENT_PAGEFAULT event: ");
447 printf("flags = %"PRIx64"; ", msg.arg.pagefault.flags);
448 printf("address = %"PRIx64"\n", msg.arg.pagefault.address);
449 /* Copy the page pointed to by 'page' into the faulting
451 region. Vary the contents that are copied in, so that it
452 is more obvious that each fault is handled separately. */
453 memset(page, 'A' + fault_cnt % 20, page_size);
455 fault_cnt++;
456 uffdio_copy.src = (unsigned long) page;
458 /* We need to handle page faults in units of pages(!).
460 So, round faulting address down to page boundary */
461 uffdio_copy.dst = (unsigned long) msg.arg.pagefault.address &
463 ~(page_size - 1);
464 uffdio_copy.len = page_size;
465 uffdio_copy.mode = 0;
466 uffdio_copy.copy = 0;
467 if (ioctl(uffd, UFFDIO_COPY, &uffdio_copy) == -1)
468 errExit("ioctl-UFFDIO_COPY");
469 printf(" (uffdio_copy.copy returned %"PRId64")\n",
471 uffdio_copy.copy);
472 }
473 }
474 int
476 main(int argc, char *argv[])
477 {
478 long uffd; /* userfaultfd file descriptor */
479 char *addr; /* Start of region handled by userfaultfd */
480 uint64_t len; /* Length of region handled by userfaultfd */
481 pthread_t thr; /* ID of thread that handles page faults */
482 struct uffdio_api uffdio_api;
483 struct uffdio_register uffdio_register;
484 int s;
485 if (argc != 2) {
487 fprintf(stderr, "Usage: %s num-pages\n", argv[0]);
488 exit(EXIT_FAILURE);
489 }
490 page_size = sysconf(_SC_PAGE_SIZE);
492 len = strtoull(argv[1], NULL, 0) * page_size;
493 /* Create and enable userfaultfd object */
495 uffd = syscall(__NR_userfaultfd, O_CLOEXEC | O_NONBLOCK);
497 if (uffd == -1)
498 errExit("userfaultfd");
499 uffdio_api.api = UFFD_API;
501 uffdio_api.features = 0;
502 if (ioctl(uffd, UFFDIO_API, &uffdio_api) == -1)
503 errExit("ioctl-UFFDIO_API");
504 /* Create a private anonymous mapping. The memory will be
506 demand-zero paged--that is, not yet allocated. When we
507 actually touch the memory, it will be allocated via
508 the userfaultfd. */
509 addr = mmap(NULL, len, PROT_READ | PROT_WRITE,
511 MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
512 if (addr == MAP_FAILED)
513 errExit("mmap");
514 printf("Address returned by mmap() = %p\n", addr);
516 /* Register the memory range of the mapping we just created for
518 handling by the userfaultfd object. In mode, we request to track
519 missing pages (i.e., pages that have not yet been faulted in). */
520 uffdio_register.range.start = (unsigned long) addr;
522 uffdio_register.range.len = len;
523 uffdio_register.mode = UFFDIO_REGISTER_MODE_MISSING;
524 if (ioctl(uffd, UFFDIO_REGISTER, &uffdio_register) == -1)
525 errExit("ioctl-UFFDIO_REGISTER");
526 /* Create a thread that will process the userfaultfd events */
528 s = pthread_create(&thr, NULL, fault_handler_thread, (void *) uffd);
530 if (s != 0) {
531 errno = s;
532 errExit("pthread_create");
533 }
534 /* Main thread now touches memory in the mapping, touching
536 locations 1024 bytes apart. This will trigger userfaultfd
537 events for all pages in the region. */
538 int l;
540 l = 0xf; /* Ensure that faulting address is not on a page
541 boundary, in order to test that we correctly
542 handle that case in fault_handling_thread() */
543 while (l < len) {
544 char c = addr[l];
545 printf("Read address %p in main(): ", addr + l);
546 printf("%c\n", c);
547 l += 1024;
548 usleep(100000); /* Slow things down a little */
549 }
550 exit(EXIT_SUCCESS);
552 }
553
555 fcntl(2), ioctl(2), ioctl_userfaultfd(2), madvise(2), mmap(2)
556 Documentation/admin-guide/mm/userfaultfd.rst in the Linux kernel source
558 tree
559
561 This page is part of release 5.10 of the Linux man-pages project. A
562 description of the project, information about reporting bugs, and the
563 latest version of this page, can be found at
564 https://www.kernel.org/doc/man-pages/.
565Linux 2020-11-01 USERFAULTFD(2)