Provided by: manpages-dev_6.8-2_all bug

NAME

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

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <fcntl.h>             /* Definition of O_* constants */
       #include <sys/syscall.h>       /* Definition of SYS_* constants */
       #include <linux/userfaultfd.h> /* Definition of UFFD_* constants */
       #include <unistd.h>

       int syscall(SYS_userfaultfd, int flags);

       Note: glibc provides no wrapper for userfaultfd(), necessitating the use of syscall(2).

DESCRIPTION

       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():

       O_CLOEXEC
              Enable the close-on-exec flag for the new userfaultfd  file  descriptor.   See  the
              description of the O_CLOEXEC flag in open(2).

       O_NONBLOCK
              Enables  non-blocking operation for the userfaultfd object.  See the description of
              the O_NONBLOCK flag in open(2).

       UFFD_USER_MODE_ONLY
              This is an userfaultfd-specific flag that was introduced in Linux 5.11.  When  set,
              the  userfaultfd object will only be able to handle page faults originated from the
              user space on the registered regions.  When a kernel-originated fault was triggered
              on the registered range with this userfaultfd, a SIGBUS signal will be delivered.

       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.

       Userfaultfd supports three modes of registration:

       UFFDIO_REGISTER_MODE_MISSING (since Linux 4.10)
              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_MINOR (since Linux 5.13)
              When registered with UFFDIO_REGISTER_MODE_MINOR mode,  user-space  will  receive  a
              page-fault  notification  when  a minor page fault occurs.  That is, when a backing
              page is in the page cache, but page table entries don't  yet  exist.   The  faulted
              thread  will  be stopped from execution until the page fault is resolved from user-
              space by an UFFDIO_CONTINUE ioctl.

       UFFDIO_REGISTER_MODE_WP (since Linux 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.

   Usage
       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 two-step handshake
       between the kernel and user space to determine what API version and  features  the  kernel
       supports,  and  then  to  enable  those features user space wants.  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 various (e.g., UFFDIO_COPY,
       UFFDIO_ZEROPAGE, or UFFDIO_CONTINUE) 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 Linux 5.7)
       Since Linux 5.7, userfaultfd supports write-protect mode for anonymous memory.   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.

       Since Linux 5.19, the write-protection mode was also  supported  on  shmem  and  hugetlbfs
       memory types.  It can be detected with the feature bit UFFD_FEATURE_WP_HUGETLBFS_SHMEM.

       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.

   Userfaultfd minor fault mode (since Linux 5.13)
       Since Linux 5.13, userfaultfd supports minor fault mode.  In this mode, fault messages are
       produced not for major faults (where the page was missing), but rather for  minor  faults,
       where  a  page  exists  in the page cache, but the page table entries are not yet present.
       The user needs to first check availability of this feature using the UFFDIO_API ioctl with
       the  appropriate  feature bits set before using this feature: UFFD_FEATURE_MINOR_HUGETLBFS
       since Linux 5.13, or UFFD_FEATURE_MINOR_SHMEM since Linux 5.14.

       To  register  with  userfaultfd  minor  fault  mode,  the  user  needs  to  initiate   the
       UFFDIO_REGISTER ioctl with mode UFFD_REGISTER_MODE_MINOR set.

       When  a  minor  fault  occurs,  user-space  will  receive  a page-fault notification whose
       uffd_msg.pagefault.flags will have the UFFD_PAGEFAULT_FLAG_MINOR flag set.

       To resolve a minor page fault, the handler should decide whether or not the existing  page
       contents  need  to  be  modified first.  If so, this should be done in-place via a second,
       non-userfaultfd-registered mapping to the same backing page (e.g., by mapping the shmem or
       hugetlbfs  file  twice).   Once  the  page  is  considered  "up to date", the fault can be
       resolved by initiating an UFFDIO_CONTINUE ioctl, which installs the page table entries and
       (by default) wakes up the faulting thread(s).

       Minor fault mode supports only hugetlbfs-backed (since Linux 5.13) and shmem-backed (since
       Linux 5.14) 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
                     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.

       pagefault.address
              The address that triggered the page fault.

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

              UFFD_PAGEFAULT_FLAG_WP
                     If this flag is set, then the fault was a write-protect fault.

              UFFD_PAGEFAULT_FLAG_MINOR
                     If this flag is set, then the fault was a minor fault.

              UFFD_PAGEFAULT_FLAG_WRITE
                     If this flag is set, then the fault was a write fault.

              If neither UFFD_PAGEFAULT_FLAG_WP nor UFFD_PAGEFAULT_FLAG_MINOR are set,  then  the
              fault was a missing fault.

       pagefault.feat.pid
              The thread ID that triggered the page fault.

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

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

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

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

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

       remove.end
              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)
              operation

       If the O_NONBLOCK flag is enabled in the associated open file description, the userfaultfd
       file descriptor can be monitored with poll(2), select(2), and epoll(7).  When  events  are
       available,  the  file  descriptor  indicates  as  readable.  If the O_NONBLOCK flag is not
       enabled, then poll(2) (always) indicates the file  as  having  a  POLLERR  condition,  and
       select(2) indicates the file descriptor as both readable and writable.

RETURN VALUE

       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.

ERRORS

       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.

STANDARDS

       Linux.

HISTORY

       Linux 4.3.

       Support for hugetlbfs and shared memory areas and non-page-fault events was added in Linux
       4.11

NOTES

       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.

BUGS

       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.

EXAMPLES

       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

           fault_handler_thread():
               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

           fault_handler_thread():
               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

           fault_handler_thread():
               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 <err.h>
       #include <errno.h>
       #include <fcntl.h>
       #include <inttypes.h>
       #include <linux/userfaultfd.h>
       #include <poll.h>
       #include <pthread.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <string.h>
       #include <sys/ioctl.h>
       #include <sys/mman.h>
       #include <sys/syscall.h>
       #include <unistd.h>

       static int page_size;

       static void *
       fault_handler_thread(void *arg)
       {
           int                 nready;
           long                uffd;   /* userfaultfd file descriptor */
           ssize_t             nread;
           struct pollfd       pollfd;
           struct uffdio_copy  uffdio_copy;

           static int      fault_cnt = 0; /* Number of faults so far handled */
           static char     *page = NULL;
           static struct uffd_msg  msg;  /* Data read from userfaultfd */

           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)
                   err(EXIT_FAILURE, "mmap");
           }

           /* Loop, handling incoming events on the userfaultfd
              file descriptor. */

           for (;;) {

               /* See what poll() tells us about the userfaultfd. */

               pollfd.fd = uffd;
               pollfd.events = POLLIN;
               nready = poll(&pollfd, 1, -1);
               if (nready == -1)
                   err(EXIT_FAILURE, "poll");

               printf("\nfault_handler_thread():\n");
               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");
                   exit(EXIT_FAILURE);
               }

               if (nread == -1)
                   err(EXIT_FAILURE, "read");

               /* We expect only one kind of event; verify that assumption. */

               if (msg.event != UFFD_EVENT_PAGEFAULT) {
                   fprintf(stderr, "Unexpected event on userfaultfd\n");
                   exit(EXIT_FAILURE);
               }

               /* 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);
               fault_cnt++;

               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)
                   err(EXIT_FAILURE, "ioctl-UFFDIO_COPY");

               printf("        (uffdio_copy.copy returned %"PRId64")\n",
                      uffdio_copy.copy);
           }
       }

       int
       main(int argc, char *argv[])
       {
           int        s;
           char       c;
           char       *addr;   /* Start of region handled by userfaultfd */
           long       uffd;    /* userfaultfd file descriptor */
           size_t     len, l;  /* 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;

           if (argc != 2) {
               fprintf(stderr, "Usage: %s num-pages\n", argv[0]);
               exit(EXIT_FAILURE);
           }

           page_size = sysconf(_SC_PAGE_SIZE);
           len = strtoull(argv[1], NULL, 0) * page_size;

           /* Create and enable userfaultfd object. */

           uffd = syscall(SYS_userfaultfd, O_CLOEXEC | O_NONBLOCK);
           if (uffd == -1)
               err(EXIT_FAILURE, "userfaultfd");

           /* NOTE: Two-step feature handshake is not needed here, since this
              example doesn't require any specific features.

              Programs that *do* should call UFFDIO_API twice: once with
              `features = 0` to detect features supported by this kernel, and
              again with the subset of features the program actually wants to
              enable. */
           uffdio_api.api = UFFD_API;
           uffdio_api.features = 0;
           if (ioctl(uffd, UFFDIO_API, &uffdio_api) == -1)
               err(EXIT_FAILURE, "ioctl-UFFDIO_API");

           /* 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)
               err(EXIT_FAILURE, "mmap");

           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)
               err(EXIT_FAILURE, "ioctl-UFFDIO_REGISTER");

           /* Create a thread that will process the userfaultfd events. */

           s = pthread_create(&thr, NULL, fault_handler_thread, (void *) uffd);
           if (s != 0) {
               errc(EXIT_FAILURE, s, "pthread_create");
           }

           /* Main thread now touches memory in the mapping, touching
              locations 1024 bytes apart. This will trigger userfaultfd
              events for all pages in the region. */

           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) {
               c = addr[l];
               printf("Read address %p in %s(): ", addr + l, __func__);
               printf("%c\n", c);
               l += 1024;
               usleep(100000);         /* Slow things down a little */
           }

           exit(EXIT_SUCCESS);
       }

SEE ALSO

       fcntl(2), ioctl(2), ioctl_userfaultfd(2), madvise(2), mmap(2)

       Documentation/admin-guide/mm/userfaultfd.rst in the Linux kernel source tree