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man:userfaultfd

USERFAULTFD(2) Linux Programmer's Manual USERFAULTFD(2)

NAME

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

SYNOPSIS

     #include <sys/types.h>
     #include <linux/userfaultfd.h>
     int userfaultfd(int flags);
     Note: There is no glibc wrapper for this system call; see NOTES.

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 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
     fcntl(2).
     The  following values may be bitwise ORed in flags to change the behav-
     ior 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).
     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.
 Usage
     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
     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-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
     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  UFF-
     DIO_COPY  or UFFDIO_ZERO ioctl(2) operations to resolve the page fault.
     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
     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_user-
     faultfd(2).
     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
     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 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
                   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_WRITE
                   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.
     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 mad-
            vise(2) or unmapped
     remove.end
            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
     errors:
     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.

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
     appropriately.

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.

VERSIONS

     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

CONFORMING TO

     userfaultfd() is Linux-specific and should  not  be  used  in  programs
     intended to be portable.

NOTES

     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  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.

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  user-
     faultfd  descriptor  might  be  created.   In  this  case,  a  spurious
     UFFD_EVENT_FORK will be delivered to the userfaultfd monitor.

EXAMPLE

     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
     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  <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)
                 errExit("mmap");
         }
         /* 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;
             pollfd.events = POLLIN;
             nready = poll(&pollfd, 1, -1);
             if (nready == -1)
                 errExit("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)
                 errExit("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 = %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);
             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)
                 errExit("ioctl-UFFDIO_COPY");
             printf("        (uffdio_copy.copy returned %lld)\n",
                     uffdio_copy.copy);
         } }
     int 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]);
             exit(EXIT_FAILURE);
         }
         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)
             errExit("userfaultfd");
         uffdio_api.api = UFFD_API;
         uffdio_api.features = 0;
         if (ioctl(uffd, UFFDIO_API, &uffdio_api) == -1)
             errExit("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)
             errExit("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)
             errExit("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) {
             errno = s;
             errExit("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. */
         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 */
         }
         exit(EXIT_SUCCESS); }

SEE ALSO

     fcntl(2), ioctl(2), ioctl_userfaultfd(2), madvise(2), mmap(2)
     Documentation/vm/userfaultfd.txt in the Linux kernel source tree

COLOPHON

     This page is part of release 4.16 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
     https://www.kernel.org/doc/man-pages/.

Linux 2017-09-15 USERFAULTFD(2)

/data/webs/external/dokuwiki/data/pages/man/userfaultfd.txt · Last modified: 2019/05/17 09:47 by 127.0.0.1

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