Provided by: manpages-dev_5.13-1_all bug


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


       #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

       Userfaultfd supports two modes of registration:

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

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

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

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

       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 ioctl_userfaultfd(2).

       Since Linux 4.11, events other than page-fault may enabled during UFFDIO_API operation.

       Up  to  Linux  4.11,  userfaultfd can be used only with anonymous private memory mappings.
       Since Linux 4.11, userfaultfd can be also used with hugetlbfs and shared memory mappings.

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

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

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

       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

              UFFD_EVENT_FORK (since Linux 4.11)
                     Generated when the faulting process invokes fork(2) (or clone(2) without the
                     CLONE_VM flag).  The event details are available in the fork field.

              UFFD_EVENT_REMAP (since Linux 4.11)
                     Generated  when  the  faulting process invokes mremap(2).  The event details
                     are available in the remap field.

              UFFD_EVENT_REMOVE (since Linux 4.11)
                     Generated when the faulting process invokes madvise(2) with MADV_DONTNEED or
                     MADV_REMOVE advice.  The event details are available in the remove field.

              UFFD_EVENT_UNMAP (since Linux 4.11)
                     Generated when the faulting process unmaps a memory range, either explicitly
                     using munmap(2) or  implicitly  during  mmap(2)  or  mremap(2).   The  event
                     details are available in the remove field.

              The address that triggered the page fault.

              A  bit  mask  of  flags  that  describe  the  event.  For UFFD_EVENT_PAGEFAULT, the
              following flag may appear:

                     If  the  address  is   in   a   range   that   was   registered   with   the
                     UFFDIO_REGISTER_MODE_MISSING  flag  (see ioctl_userfaultfd(2)) and this flag
                     is set, this a write fault; otherwise it is a read fault.

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

              The file descriptor associated with the userfault  object  created  for  the  child
              created by fork(2).

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

              The original length of the memory range that was remapped using mremap(2).

              The start address of the memory range that was freed using madvise(2) or unmapped

              The end address of the memory range that was freed using madvise(2) or unmapped

       A read(2) on a userfaultfd file descriptor can fail with the following errors:

       EINVAL The  userfaultfd  object  has  not  yet  been enabled using the UFFDIO_API ioctl(2)

       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 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 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.


       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


       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 <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 tree


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       project,  information  about  reporting  bugs, and the latest version of this page, can be
       found at