noble (2) userfaultfd.2.gz

Provided by: manpages-dev_6.7-2_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