userfaultfd(2) — Linux manual page


USERFAULTFD(2)          Linux Programmer's Manual         USERFAULTFD(2)

NAME         top

       userfaultfd - create a file descriptor for handling page faults
       in user space

SYNOPSIS         top

       #include <sys/types.h>
       #include <linux/userfaultfd.h>

       int userfaultfd(int flags);

       Note: There is no glibc wrapper for this system call; see NOTES.

DESCRIPTION         top

       userfaultfd() creates a new userfaultfd object that can be used
       for delegation of page-fault handling to a user-space
       application, and returns a file descriptor that refers to the new
       object.  The new userfaultfd object is configured using ioctl(2).

       Once the userfaultfd object is configured, the application can
       use read(2) to receive userfaultfd notifications.  The reads from
       userfaultfd may be blocking or non-blocking, depending on the
       value of flags used for the creation of the userfaultfd or
       subsequent calls to fcntl(2).

       The following values may be bitwise ORed in flags to change the
       behavior of userfaultfd():

              Enable the close-on-exec flag for the new userfaultfd file
              descriptor.  See the description of the O_CLOEXEC flag in

              Enables non-blocking operation for the userfaultfd object.
              See the description of the O_NONBLOCK flag in open(2).

       When the last file descriptor referring to a userfaultfd object
       is closed, all memory ranges that were registered with the object
       are unregistered and unread events are flushed.

       The userfaultfd mechanism is designed to allow a thread in a
       multithreaded program to perform user-space paging for the other
       threads in the process.  When a page fault occurs for one of the
       regions registered to the userfaultfd object, the faulting thread
       is put to sleep and an event is generated that can be read via
       the userfaultfd file descriptor.  The fault-handling thread reads
       events from this file descriptor and services them using the
       operations described in ioctl_userfaultfd(2).  When servicing the
       page fault events, the fault-handling thread can trigger a wake-
       up for the sleeping thread.

       It is possible for the faulting threads and the fault-handling
       threads to run in the context of different processes.  In this
       case, these threads may belong to different programs, and the
       program that executes the faulting threads will not necessarily
       cooperate with the program that handles the page faults.  In such
       non-cooperative mode, the process that monitors userfaultfd and
       handles page faults needs to be aware of the changes in the
       virtual memory layout of the faulting process to avoid memory

       Starting from Linux 4.11, userfaultfd can also notify the fault-
       handling threads about changes in the virtual memory layout of
       the faulting process.  In addition, if the faulting process
       invokes fork(2), the userfaultfd objects associated with the
       parent may be duplicated into the child process and the
       userfaultfd monitor will be notified (via the UFFD_EVENT_FORK
       described below) about the file descriptor associated with the
       userfault objects created for the child process, which allows the
       userfaultfd monitor to perform user-space paging for the child
       process.  Unlike page faults which have to be synchronous and
       require an explicit or implicit wakeup, all other events are
       delivered asynchronously and the non-cooperative process resumes
       execution as soon as the userfaultfd manager executes read(2).
       The userfaultfd manager should carefully synchronize calls to
       UFFDIO_COPY with the processing of events.

       The current asynchronous model of the event delivery is optimal
       for single threaded non-cooperative userfaultfd manager

   Userfaultfd operation
       After the userfaultfd object is created with userfaultfd(), the
       application must enable it using the UFFDIO_API ioctl(2)
       operation.  This operation allows a handshake between the kernel
       and user space to determine the API version and supported
       features.  This operation must be performed before any of the
       other ioctl(2) operations described below (or those operations
       fail with the EINVAL error).

       After a successful UFFDIO_API operation, the application then
       registers memory address ranges using the UFFDIO_REGISTER
       ioctl(2) operation.  After successful completion of a
       UFFDIO_REGISTER operation, a page fault occurring in the
       requested memory range, and satisfying the mode defined at the
       registration time, will be forwarded by the kernel to the user-
       space application.  The application can then use the UFFDIO_COPY
       or UFFDIO_ZEROPAGE ioctl(2) operations to resolve the page fault.

       Starting from Linux 4.14, if the application sets the
       UFFD_FEATURE_SIGBUS feature bit using the UFFDIO_API ioctl(2), no
       page-fault notification will be forwarded to user space.  Instead
       a SIGBUS signal is delivered to the faulting process.  With this
       feature, userfaultfd can be used for robustness purposes to
       simply catch any access to areas within the registered address
       range that do not have pages allocated, without having to listen
       to userfaultfd events.  No userfaultfd monitor will be required
       for dealing with such memory accesses.  For example, this feature
       can be useful for applications that want to prevent the kernel
       from automatically allocating pages and filling holes in sparse
       files when the hole is accessed through a memory mapping.

       The UFFD_FEATURE_SIGBUS feature is implicitly inherited through
       fork(2) if used in combination with UFFD_FEATURE_FORK.

       Details of the various ioctl(2) operations can be found in

       Since Linux 4.11, events other than page-fault may enabled during
       UFFDIO_API operation.

       Up to Linux 4.11, userfaultfd can be used only with anonymous
       private memory mappings.  Since Linux 4.11, userfaultfd can be
       also used with hugetlbfs and shared memory mappings.

   Reading from the userfaultfd structure
       Each read(2) from the userfaultfd file descriptor returns one or
       more uffd_msg structures, each of which describes a page-fault
       event or an event required for the non-cooperative userfaultfd

           struct uffd_msg {
               __u8  event;            /* Type of event */
               union {
                   struct {
                       __u64 flags;    /* Flags describing fault */
                       __u64 address;  /* Faulting address */
                   } pagefault;

                   struct {            /* Since Linux 4.11 */
                       __u32 ufd;      /* Userfault file descriptor
                                          of the child process */
                   } fork;

                   struct {            /* Since Linux 4.11 */
                       __u64 from;     /* Old address of remapped area */
                       __u64 to;       /* New address of remapped area */
                       __u64 len;      /* Original mapping length */
                   } remap;

                   struct {            /* Since Linux 4.11 */
                       __u64 start;    /* Start address of removed area */
                       __u64 end;      /* End address of removed area */
                   } remove;
               } arg;

               /* Padding fields omitted */
           } __packed;

       If multiple events are available and the supplied buffer is large
       enough, read(2) returns as many events as will fit in the
       supplied buffer.  If the buffer supplied to read(2) is smaller
       than the size of the uffd_msg structure, the read(2) fails with
       the error EINVAL.

       The fields set in the uffd_msg structure are as follows:

       event  The type of event.  Depending of the event type, different
              fields of the arg union represent details required for the
              event processing.  The non-page-fault events are generated
              only when appropriate feature is enabled during API
              handshake with UFFDIO_API ioctl(2).

              The following values can appear in the event field:

              UFFD_EVENT_PAGEFAULT (since Linux 4.3)
                     A page-fault event.  The page-fault details are
                     available in the pagefault field.

              UFFD_EVENT_FORK (since Linux 4.11)
                     Generated when the faulting process invokes fork(2)
                     (or clone(2) without the CLONE_VM flag).  The event
                     details are available in the fork field.

              UFFD_EVENT_REMAP (since Linux 4.11)
                     Generated when the faulting process invokes
                     mremap(2).  The event details are available in the
                     remap field.

              UFFD_EVENT_REMOVE (since Linux 4.11)
                     Generated when the faulting process invokes
                     madvise(2) with MADV_DONTNEED or MADV_REMOVE
                     advice.  The event details are available in the
                     remove field.

              UFFD_EVENT_UNMAP (since Linux 4.11)
                     Generated when the faulting process unmaps a memory
                     range, either explicitly using munmap(2) or
                     implicitly during mmap(2) or mremap(2).  The event
                     details are available in the remove field.

              The address that triggered the page fault.

              A bit mask of flags that describe the event.  For
              UFFD_EVENT_PAGEFAULT, the following flag may appear:

                     If the address is in a range that was registered
                     with the UFFDIO_REGISTER_MODE_MISSING flag (see
                     ioctl_userfaultfd(2)) and this flag is set, this a
                     write fault; otherwise it is a read fault.

              The file descriptor associated with the userfault object
              created for the child created by fork(2).

              The original address of the memory range that was remapped
              using mremap(2).
              The new address of the memory range that was remapped
              using mremap(2).

              The original length of the memory range that was remapped
              using mremap(2).

              The start address of the memory range that was freed using
              madvise(2) or unmapped

              The end address of the memory range that was freed using
              madvise(2) or unmapped

       A read(2) on a userfaultfd file descriptor can fail with the
       following errors:

       EINVAL The userfaultfd object has not yet been enabled using the
              UFFDIO_API ioctl(2) operation

       If the O_NONBLOCK flag is enabled in the associated open file
       description, the userfaultfd file descriptor can be monitored
       with poll(2), select(2), and epoll(7).  When events are
       available, the file descriptor indicates as readable.  If the
       O_NONBLOCK flag is not enabled, then poll(2) (always) indicates
       the file as having a POLLERR condition, and select(2) indicates
       the file descriptor as both readable and writable.

RETURN VALUE         top

       On success, userfaultfd() returns a new file descriptor that
       refers to the userfaultfd object.  On error, -1 is returned, and
       errno is set to indicate the error.

ERRORS         top

       EINVAL An unsupported value was specified in flags.

       EMFILE The per-process limit on the number of open file
              descriptors has been reached

       ENFILE The system-wide limit on the total number of open files
              has been reached.

       ENOMEM Insufficient kernel memory was available.

       EPERM (since Linux 5.2)
              The caller is not privileged (does not have the
              CAP_SYS_PTRACE capability in the initial user namespace),
              and /proc/sys/vm/unprivileged_userfaultfd has the value 0.

VERSIONS         top

       The userfaultfd() system call first appeared in Linux 4.3.

       The support for hugetlbfs and shared memory areas and non-page-
       fault events was added in Linux 4.11

CONFORMING TO         top

       userfaultfd() is Linux-specific and should not be used in
       programs intended to be portable.

NOTES         top

       Glibc does not provide a wrapper for this system call; call it
       using syscall(2).

       The userfaultfd mechanism can be used as an alternative to
       traditional user-space paging techniques based on the use of the
       SIGSEGV signal and mmap(2).  It can also be used to implement
       lazy restore for checkpoint/restore mechanisms, as well as post-
       copy migration to allow (nearly) uninterrupted execution when
       transferring virtual machines and Linux containers from one host
       to another.

BUGS         top

       If the UFFD_FEATURE_EVENT_FORK is enabled and a system call from
       the fork(2) family is interrupted by a signal or failed, a stale
       userfaultfd descriptor might be created.  In this case, a
       spurious UFFD_EVENT_FORK will be delivered to the userfaultfd

EXAMPLES         top

       The program below demonstrates the use of the userfaultfd
       mechanism.  The program creates two threads, one of which acts as
       the page-fault handler for the process, for the pages in a
       demand-page zero region created using mmap(2).

       The program takes one command-line argument, which is the number
       of pages that will be created in a mapping whose page faults will
       be handled via userfaultfd.  After creating a userfaultfd object,
       the program then creates an anonymous private mapping of the
       specified size and registers the address range of that mapping
       using the UFFDIO_REGISTER ioctl(2) operation.  The program then
       creates a second thread that will perform the task of handling
       page faults.

       The main thread then walks through the pages of the mapping
       fetching bytes from successive pages.  Because the pages have not
       yet been accessed, the first access of a byte in each page will
       trigger a page-fault event on the userfaultfd file descriptor.

       Each of the page-fault events is handled by the second thread,
       which sits in a loop processing input from the userfaultfd file
       descriptor.  In each loop iteration, the second thread first
       calls poll(2) to check the state of the file descriptor, and then
       reads an event from the file descriptor.  All such events should
       be UFFD_EVENT_PAGEFAULT events, which the thread handles by
       copying a page of data into the faulting region using the
       UFFDIO_COPY ioctl(2) operation.

       The following is an example of what we see when running the

           $ ./userfaultfd_demo 3
           Address returned by mmap() = 0x7fd30106c000

               poll() returns: nready = 1; POLLIN = 1; POLLERR = 0
               UFFD_EVENT_PAGEFAULT event: flags = 0; address = 7fd30106c00f
                   (uffdio_copy.copy returned 4096)
           Read address 0x7fd30106c00f in main(): A
           Read address 0x7fd30106c40f in main(): A
           Read address 0x7fd30106c80f in main(): A
           Read address 0x7fd30106cc0f in main(): A

               poll() returns: nready = 1; POLLIN = 1; POLLERR = 0
               UFFD_EVENT_PAGEFAULT event: flags = 0; address = 7fd30106d00f
                   (uffdio_copy.copy returned 4096)
           Read address 0x7fd30106d00f in main(): B
           Read address 0x7fd30106d40f in main(): B
           Read address 0x7fd30106d80f in main(): B
           Read address 0x7fd30106dc0f in main(): B

               poll() returns: nready = 1; POLLIN = 1; POLLERR = 0
               UFFD_EVENT_PAGEFAULT event: flags = 0; address = 7fd30106e00f
                   (uffdio_copy.copy returned 4096)
           Read address 0x7fd30106e00f in main(): C
           Read address 0x7fd30106e40f in main(): C
           Read address 0x7fd30106e80f in main(): C
           Read address 0x7fd30106ec0f in main(): C

   Program source

       /* userfaultfd_demo.c

          Licensed under the GNU General Public License version 2 or later.
       #define _GNU_SOURCE
       #include <inttypes.h>
       #include <sys/types.h>
       #include <stdio.h>
       #include <linux/userfaultfd.h>
       #include <pthread.h>
       #include <errno.h>
       #include <unistd.h>
       #include <stdlib.h>
       #include <fcntl.h>
       #include <signal.h>
       #include <poll.h>
       #include <string.h>
       #include <sys/mman.h>
       #include <sys/syscall.h>
       #include <sys/ioctl.h>
       #include <poll.h>

       #define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \
                               } while (0)

       static int page_size;

       static void *
       fault_handler_thread(void *arg)
           static struct uffd_msg msg;   /* Data read from userfaultfd */
           static int fault_cnt = 0;     /* Number of faults so far handled */
           long uffd;                    /* userfaultfd file descriptor */
           static char *page = NULL;
           struct uffdio_copy uffdio_copy;
           ssize_t nread;

           uffd = (long) arg;

           /* Create a page that will be copied into the faulting region. */

           if (page == NULL) {
               page = mmap(NULL, page_size, PROT_READ | PROT_WRITE,
                           MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
               if (page == MAP_FAILED)

           /* Loop, handling incoming events on the userfaultfd
              file descriptor. */

           for (;;) {

               /* See what poll() tells us about the userfaultfd. */

               struct pollfd pollfd;
               int nready;
               pollfd.fd = uffd;
      = POLLIN;
               nready = poll(&pollfd, 1, -1);
               if (nready == -1)

               printf("    poll() returns: nready = %d; "
                       "POLLIN = %d; POLLERR = %d\n", nready,
                       (pollfd.revents & POLLIN) != 0,
                       (pollfd.revents & POLLERR) != 0);

               /* Read an event from the userfaultfd. */

               nread = read(uffd, &msg, sizeof(msg));
               if (nread == 0) {
                   printf("EOF on userfaultfd!\n");

               if (nread == -1)

               /* We expect only one kind of event; verify that assumption. */

               if (msg.event != UFFD_EVENT_PAGEFAULT) {
                   fprintf(stderr, "Unexpected event on userfaultfd\n");

               /* Display info about the page-fault event. */

               printf("    UFFD_EVENT_PAGEFAULT event: ");
               printf("flags = %"PRIx64"; ", msg.arg.pagefault.flags);
               printf("address = %"PRIx64"\n", msg.arg.pagefault.address);

               /* Copy the page pointed to by 'page' into the faulting
                  region. Vary the contents that are copied in, so that it
                  is more obvious that each fault is handled separately. */

               memset(page, 'A' + fault_cnt % 20, page_size);

               uffdio_copy.src = (unsigned long) page;

               /* We need to handle page faults in units of pages(!).
                  So, round faulting address down to page boundary. */

               uffdio_copy.dst = (unsigned long) msg.arg.pagefault.address &
                                                  ~(page_size - 1);
               uffdio_copy.len = page_size;
               uffdio_copy.mode = 0;
               uffdio_copy.copy = 0;
               if (ioctl(uffd, UFFDIO_COPY, &uffdio_copy) == -1)

               printf("        (uffdio_copy.copy returned %"PRId64")\n",

       main(int argc, char *argv[])
           long uffd;          /* userfaultfd file descriptor */
           char *addr;         /* Start of region handled by userfaultfd */
           uint64_t len;       /* Length of region handled by userfaultfd */
           pthread_t thr;      /* ID of thread that handles page faults */
           struct uffdio_api uffdio_api;
           struct uffdio_register uffdio_register;
           int s;

           if (argc != 2) {
               fprintf(stderr, "Usage: %s num-pages\n", argv[0]);

           page_size = sysconf(_SC_PAGE_SIZE);
           len = strtoull(argv[1], NULL, 0) * page_size;

           /* Create and enable userfaultfd object. */

           uffd = syscall(__NR_userfaultfd, O_CLOEXEC | O_NONBLOCK);
           if (uffd == -1)

           uffdio_api.api = UFFD_API;
           uffdio_api.features = 0;
           if (ioctl(uffd, UFFDIO_API, &uffdio_api) == -1)

           /* Create a private anonymous mapping. The memory will be
              demand-zero paged--that is, not yet allocated. When we
              actually touch the memory, it will be allocated via
              the userfaultfd. */

           addr = mmap(NULL, len, PROT_READ | PROT_WRITE,
                       MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
           if (addr == MAP_FAILED)

           printf("Address returned by mmap() = %p\n", addr);

           /* Register the memory range of the mapping we just created for
              handling by the userfaultfd object. In mode, we request to track
              missing pages (i.e., pages that have not yet been faulted in). */

           uffdio_register.range.start = (unsigned long) addr;
           uffdio_register.range.len = len;
           uffdio_register.mode = UFFDIO_REGISTER_MODE_MISSING;
           if (ioctl(uffd, UFFDIO_REGISTER, &uffdio_register) == -1)

           /* Create a thread that will process the userfaultfd events. */

           s = pthread_create(&thr, NULL, fault_handler_thread, (void *) uffd);
           if (s != 0) {
               errno = s;

           /* Main thread now touches memory in the mapping, touching
              locations 1024 bytes apart. This will trigger userfaultfd
              events for all pages in the region. */

           int l;
           l = 0xf;    /* Ensure that faulting address is not on a page
                          boundary, in order to test that we correctly
                          handle that case in fault_handling_thread(). */
           while (l < len) {
               char c = addr[l];
               printf("Read address %p in main(): ", addr + l);
               printf("%c\n", c);
               l += 1024;
               usleep(100000);         /* Slow things down a little */


SEE ALSO         top

       fcntl(2), ioctl(2), ioctl_userfaultfd(2), madvise(2), mmap(2)

       Documentation/admin-guide/mm/userfaultfd.rst in the Linux kernel
       source tree

COLOPHON         top

       This page is part of release 5.11 of the Linux man-pages project.
       A description of the project, information about reporting bugs,
       and the latest version of this page, can be found at

Linux                          2021-03-22                 USERFAULTFD(2)

Pages that refer to this page: ioctl_userfaultfd(2)mmap(2)mremap(2)syscalls(2)proc(5)