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

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

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

       int userfaultfd(int flags);

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

       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

       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

       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 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
              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 writable.

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

       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.

       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.

       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 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 <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 = %llx; ", msg.arg.pagefault.flags);
               printf("address = %llx\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 %lld)\n",

       main(int argc, char *argv[])
           long uffd;          /* userfaultfd file descriptor */
           char *addr;         /* Start of region handled by userfaultfd */
           unsigned long 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 = strtoul(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 5.07 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                             2020-06-09                    USERFAULTFD(2)