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 <fcntl.h>            /* Definition of O_* constants */
       #include <sys/syscall.h>      /* Definition of SYS_* constants */
       #include <unistd.h>

       int syscall(SYS_userfaultfd, int flags);

       Note: glibc provides no wrapper for userfaultfd(), necessitating
       the use of syscall(2).

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.

       Userfaultfd supports two modes of registration:

              When registered with UFFDIO_REGISTER_MODE_MISSING mode,
              user-space will receive a page-fault notification when a
              missing page is accessed.  The faulted thread will be
              stopped from execution until the page fault is resolved
              from user-space by either an UFFDIO_COPY or an
              UFFDIO_ZEROPAGE ioctl.

       UFFDIO_REGISTER_MODE_WP (since 5.7)
              When registered with UFFDIO_REGISTER_MODE_WP mode, user-
              space will receive a page-fault notification when a write-
              protected page is written.  The faulted thread will be
              stopped from execution until user-space write-unprotects
              the page using an UFFDIO_WRITEPROTECT ioctl.

       Multiple modes can be enabled at the same time for the same
       memory range.

       Since Linux 4.14, a userfaultfd page-fault notification can
       selectively embed faulting thread ID information into the
       notification.  One needs to enable this feature explicitly using
       the UFFD_FEATURE_THREAD_ID feature bit when initializing the
       userfaultfd context.  By default, thread ID reporting is

       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

       Since 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

       Since Linux 5.7, userfaultfd is able to do synchronous page dirty
       tracking using the new write-protect register mode.  One should
       check against the feature bit UFFD_FEATURE_PAGEFAULT_FLAG_WP
       before using this feature.  Similar to the original userfaultfd
       missing mode, the write-protect mode will generate a userfaultfd
       notification when the protected page is written.  The user needs
       to resolve the page fault by unprotecting the faulted page and
       kicking the faulted thread to continue.  For more information,
       please refer to the "Userfaultfd write-protect mode" section.

   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.

       Since 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.

   Userfaultfd write-protect mode (since 5.7)
       Since Linux 5.7, userfaultfd supports write-protect mode.  The
       user needs to first check availability of this feature using
       UFFDIO_API ioctl against the feature bit
       UFFD_FEATURE_PAGEFAULT_FLAG_WP before using this feature.

       To register with userfaultfd write-protect mode, the user needs
       to initiate the UFFDIO_REGISTER ioctl with mode
       UFFDIO_REGISTER_MODE_WP set.  Note that it is legal to monitor
       the same memory range with multiple modes.  For example, the user
       can do UFFDIO_REGISTER with the mode set to
       there is only UFFDIO_REGISTER_MODE_WP registered, user-space will
       not receive any notification when a missing page is written.
       Instead, user-space will receive a write-protect page-fault
       notification only when an existing but write-protected page got

       After the UFFDIO_REGISTER ioctl completed with
       UFFDIO_REGISTER_MODE_WP mode set, the user can write-protect any
       existing memory within the range using the ioctl
       UFFDIO_WRITEPROTECT where uffdio_writeprotect.mode should be set

       When a write-protect event happens, user-space will receive a
       page-fault notification whose uffd_msg.pagefault.flags will be
       with UFFD_PAGEFAULT_FLAG_WP flag set.  Note: since only writes
       can trigger this kind of fault, write-protect notifications will
       always have the UFFD_PAGEFAULT_FLAG_WRITE bit set along with the

       To resolve a write-protection page fault, the user should
       initiate another UFFDIO_WRITEPROTECT ioctl, whose
       uffd_msg.pagefault.flags should have the flag
       UFFDIO_WRITEPROTECT_MODE_WP cleared upon the faulted page or

       Write-protect mode supports only private anonymous memory.

   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 */
                       union {
                           __u32 ptid; /* Thread ID of the fault */
                       } feat;
                   } 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.

                     If the address is in a range that was registered
                     with the UFFDIO_REGISTER_MODE_WP flag, when this
                     bit is set, it means it is a write-protect fault.
                     Otherwise it is a page-missing fault.
              The thread ID that triggered the page 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

       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 5.13 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)