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

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

       #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).

       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 open(2).

              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

       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 disabled.

       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 corruption.

       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

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

       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

       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
       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 written.

       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

       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

       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 range.

       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 usage:

           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
              The new address of the memory range that was remapped using

              The original length of the memory range that was remapped using

              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

       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

       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.

       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

       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.

       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

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

       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.

       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 monitor.

       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

       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 program:

           $ ./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 */


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

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

       This page is part of release 5.12 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)