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 corruption.

       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 implementations.

   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 usage:

           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 avail‐
                     able 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 cre‐
              ated for the child created by fork(2).

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

              The original length of the memory range that was remapped us‐
              ing 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 follow‐
       ing errors:

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

       If the O_NONBLOCK flag is enabled in the associated open file de‐
       scription, 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 read‐
       able 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 appropriately.

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

       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 monitor.

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 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 */


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 5.09 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                            2020-11-01                   USERFAULTFD(2)

Pages that refer to this page: ioctl_userfaultfd(2)mmap(2)mremap(2)munmap(2)syscalls(2)mmap64(3)proc(5)procfs(5)