Date:   Mon, 20 Mar 2017 21:08:05 +0100
From:   "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:     Andrea Arcangeli <aarcange@...hat.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>
Cc:     mtk.manpages@...il.com, lkml <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        linux-man <linux-man@...r.kernel.org>
Subject: Review request: draft userfaultfd(2) manual page

Hello Andrea, Mike, and all,

Mike: thanks for the page that you sent. I've reworked it
a bit, and also added a lot of further information,
and an example program. In the process, I split the page
into two pieces, with one piece describing the userfaultfd()
system call and the other describing the ioctl() operations.

I'd like to get review input, especially from you and
Andrea, but also anyone else, for the current version
of this page, which includes a few FIXMEs to be sorted.

I've shown the rendered version of the page below. 
The groff source is attached, and can also be found
at the branch here:

https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/log/?h=draft_userfaultfd

The new ioctl_userfaultfd(2) page follows this mail.

Cheers,

Michael


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

┌─────────────────────────────────────────────────────┐
│FIXME                                                │
├─────────────────────────────────────────────────────┤
│Need  to  describe close(2) semantics for userfaulfd │
│file descriptor: what happens when  the  userfaultfd │
│FD is closed?                                        │
│                                                     │
└─────────────────────────────────────────────────────┘

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  applica‐
       tion,  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).

   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 fault‐
       ing thread is put to sleep and an event is generated  that  can
       be  read  via  the userfaultfd file descriptor.  The fault-han‐
       dling thread reads events from this file  descriptor  and  ser‐
       vices  them  using  the  operations  described  in  ioctl_user‐
       faultfd(2).  When servicing the page fault events,  the  fault-
       handling thread can trigger a wake-up for the sleeping thread.

   Userfaultfd operation
       After the userfaultfd object is created with userfaultfd(), the
       application must enable it using the UFFDIO_API ioctl(2) opera‐
       tion.  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 UFF‐
       DIO_REGISTER operation, a page fault occurring in the requested
       memory  range, and satisfying the mode defined at the registra‐
       tion 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.

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

       Currently,  userfaultfd can be used only with anonymous private
       memory mappings.

   Reading from the userfaultfd structure
       ┌─────────────────────────────────────────────────────┐
       │FIXME                                                │
       ├─────────────────────────────────────────────────────┤
       │are the details below correct?                       │
       └─────────────────────────────────────────────────────┘
       Each read(2) from the userfaultfd file descriptor  returns  one
       or  more  uffd_msg  structures, each of which describes a page-
       fault event:

           struct uffd_msg {
               __u8  event;                /* Type of event */
               ...
               union {
                   struct {
                       __u64 flags;        /* Flags describing fault */
                       __u64 address;      /* Faulting address */
                   } pagefault;
                   ...
               } 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.  Currently, only one value can appear
              in  this  field: UFFD_EVENT_PAGEFAULT, which indicates a
              page-fault event.

       address
              The address that triggered the page fault.

       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_userfaultfd(2))  and this flag is set, this
                     a write fault; otherwise it is a read fault.

       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

       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.


       ┌─────────────────────────────────────────────────────┐
       │FIXME                                                │
       ├─────────────────────────────────────────────────────┤
       │But, it seems,  the  object  must  be  created  with │
       │O_NONBLOCK.  What is the rationale for this require‐ │
       │ment? Something needs to  be  said  in  this  manual │
       │page.                                                │
       └─────────────────────────────────────────────────────┘

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.

CONFORMING TO
       userfaultfd()  is Linux-specific and should not be used in pro‐
       grams 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 tra‐
       ditional 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 from one host to another.

EXAMPLE
       The program below demonstrates the use of the userfaultfd mech‐
       anism.   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 num‐
       ber of pages that will be  created  in  a  mapping  whose  page
       faults will be handled via userfaultfd.  After creating a user‐
       faultfd 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


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/

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