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 re-
       turns 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  de-
              scriptor.  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 un-
       registered 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 de-
       scriptor.   The  fault-handling  thread reads events from this file de-
       scriptor  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 de-
       termine 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_ZEROPAGE ioctl(2) operations  to  resolve  the  page

       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 de-
       livered 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 fea-
       ture  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  de-
                     tails 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 ap-

       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/un-
              privileged_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  in-
       tended 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  ac-
       cessed,  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 5.05 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                             2020-02-09                    USERFAULTFD(2)
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