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


       #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 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

       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_ZERO
       ioctl(2) operations to resolve the page fault.

       Starting  from  Linux  4.14,  if  the application sets the UFFD_FEATURE_SIGBUS feature bit
       using the UFFDIO_API ioctl(2), no page-fault notification will be forwarded to user space.
       Instead  a  SIGBUS  signal  is  delivered  to  the  faulting  process.  With this feature,
       userfaultfd can be used for robustness purposes to simply catch any access to areas within
       the registered address range that do not have pages allocated, without having to listen to
       userfaultfd events.  No userfaultfd monitor will be required for dealing with such  memory
       accesses.   For  example, this feature can be useful for applications that want to prevent
       the kernel from automatically allocating pages and filling holes in sparse files when  the
       hole is accessed through a memory mapping.

       The  UFFD_FEATURE_SIGBUS  feature  is  implicitly  inherited  through  fork(2)  if used in
       combination with UFFD_FEATURE_FORK.

       Details of the various ioctl(2) operations can be found in 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.

   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

              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 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 appropriately.


       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.


       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


       Glibc does not provide a wrapper for this system call; call it using syscall(2).

       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/vm/userfaultfd.txt in the Linux kernel source tree


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