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 user-
       faultfd 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  user-
       faultfd  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 behav-
       ior 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 multi-
       threaded program to perform user-space paging for the other threads  in
       the  process.   When  a page fault occurs for one of the regions regis-
       tered 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-han-
       dling  threads about changes in the virtual memory layout of the fault-
       ing 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 asyn-
       chronously 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  appli-
       cation  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  UFF-
       DIO_COPY or UFFDIO_ZERO ioctl(2) operations to resolve the page fault.

       Starting from Linux 4.14, if the application sets the UFFD_FEATURE_SIG-
       BUS feature bit using the UFFDIO_API ioctl(2), no page-fault  notifica-
       tion  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  ioctl_user-

       Since  Linux 4.11, events other than page-fault may enabled during UFF-
       DIO_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 buf-
       fer.  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  UFF-
              DIO_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_user-
                     faultfd(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  mad-
              vise(2) or unmapped

              The  end  address  of the memory range that was freed using mad-
              vise(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 UFF-
              DIO_API ioctl(2) operation

       If the O_NONBLOCK flag is enabled in the associated open file  descrip-
       tion,  the  userfaultfd  file descriptor can be monitored with poll(2),
       select(2), and epoll(7).  When events are available, the file  descrip-
       tor 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.

       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  check-
       point/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  user-
       faultfd  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  han-
       dled 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/vm/userfaultfd.txt in the Linux kernel source tree

       This page is part of release 4.15 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                             2017-09-15                    USERFAULTFD(2)
Man Pages Copyright Respective Owners. Site Copyright (C) 1994 - 2022 Hurricane Electric. All Rights Reserved.