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SubjectReview 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:

The new ioctl_userfaultfd(2) page follows this mail.



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

│Need to describe close(2) semantics for userfaulfd │
│file descriptor: what happens when the userfaultfd │
│FD is closed? │
│ │

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

#include <sys/types.h>
#include <linux/userfaultfd.h>

int userfaultfd(int flags);

Note: There is no glibc wrapper for this system call; see

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

Once the userfaultfd object is configured, the application can
use read(2) to receive userfaultfd notifications. The reads
from userfaultfd may be blocking or non-blocking, depending on
the value of flags used for the creation of the userfaultfd or
subsequent calls to fcntl(2).

The following values may be bitwise ORed in flags to change the
behavior of userfaultfd():

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

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

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

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

Reading from the userfaultfd structure
│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.

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:

If the address is in a range that was registered
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.

│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. │

On success, userfaultfd() returns a new file descriptor that
refers to the userfaultfd object. On error, -1 is returned,
and errno is set appropriately.

EINVAL An unsupported value was specified in flags.

EMFILE The per-process limit on the number of open file
descriptors has been reached

ENFILE The system-wide limit on the total number of open files
has been reached.

ENOMEM Insufficient kernel memory was available.

The userfaultfd() system call first appeared in Linux 4.3.

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

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.

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

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

$ ./userfaultfd_demo 3
Address returned by mmap() = 0x7fd30106c000

poll() returns: nready = 1; POLLIN = 1; POLLERR = 0
UFFD_EVENT_PAGEFAULT event: flags = 0; address = 7fd30106c00f
(uffdio_copy.copy returned 4096)
Read address 0x7fd30106c00f in main(): A
Read address 0x7fd30106c40f in main(): A
Read address 0x7fd30106c80f in main(): A
Read address 0x7fd30106cc0f in main(): A

poll() returns: nready = 1; POLLIN = 1; POLLERR = 0
UFFD_EVENT_PAGEFAULT event: flags = 0; address = 7fd30106d00f
(uffdio_copy.copy returned 4096)
Read address 0x7fd30106d00f in main(): B
Read address 0x7fd30106d40f in main(): B
Read address 0x7fd30106d80f in main(): B
Read address 0x7fd30106dc0f in main(): B

poll() returns: nready = 1; POLLIN = 1; POLLERR = 0
UFFD_EVENT_PAGEFAULT event: flags = 0; address = 7fd30106e00f
(uffdio_copy.copy returned 4096)
Read address 0x7fd30106e00f in main(): C
Read address 0x7fd30106e40f in main(): C
Read address 0x7fd30106e80f in main(): C
Read address 0x7fd30106ec0f in main(): C

Program source

/* userfaultfd_demo.c

Licensed under the GNU General Public License version 2 or later.
#define _GNU_SOURCE
#include <sys/types.h>
#include <stdio.h>
#include <linux/userfaultfd.h>
#include <pthread.h>
#include <errno.h>
#include <unistd.h>
#include <stdlib.h>
#include <fcntl.h>
#include <signal.h>
#include <poll.h>
#include <string.h>
#include <sys/mman.h>
#include <sys/syscall.h>
#include <sys/ioctl.h>
#include <poll.h>

#define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \
} while (0)

static int page_size;

static void *
fault_handler_thread(void *arg)
static struct uffd_msg msg; /* Data read from userfaultfd */
static int fault_cnt = 0; /* Number of faults so far handled */
long uffd; /* userfaultfd file descriptor */
static char *page = NULL;
struct uffdio_copy uffdio_copy;
ssize_t nread;

uffd = (long) arg;

/* Create a page that will be copied into the faulting region */

if (page == NULL) {
page = mmap(NULL, page_size, PROT_READ | PROT_WRITE,
if (page == MAP_FAILED)

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

for (;;) {

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

struct pollfd pollfd;
int nready;
pollfd.fd = uffd; = POLLIN;
nready = poll(&pollfd, 1, -1);
if (nready == -1)

printf(" poll() returns: nready = %d; "
"POLLIN = %d; POLLERR = %d\n", nready,
(pollfd.revents & POLLIN) != 0,
(pollfd.revents & POLLERR) != 0);

/* Read an event from the userfaultfd */

nread = read(uffd, &msg, sizeof(msg));
if (nread == 0) {
printf("EOF on userfaultfd!\n");

if (nread == -1)

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

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

/* Display info about the page-fault event */

printf(" UFFD_EVENT_PAGEFAULT event: ");
printf("flags = %llx; ", msg.arg.pagefault.flags);
printf("address = %llx\n", msg.arg.pagefault.address);

/* Copy the page pointed to by 'page' into the faulting
region. Vary the contents that are copied in, so that it
is more obvious that each fault is handled separately. */

memset(page, 'A' + fault_cnt % 20, page_size);

uffdio_copy.src = (unsigned long) page;

/* We need to handle page faults in units of pages(!).
So, round faulting address down to page boundary */

uffdio_copy.dst = (unsigned long) msg.arg.pagefault.address &
~(page_size - 1);
uffdio_copy.len = page_size;
uffdio_copy.mode = 0;
uffdio_copy.copy = 0;
if (ioctl(uffd, UFFDIO_COPY, &uffdio_copy) == -1)

printf(" (uffdio_copy.copy returned %lld)\n",

main(int argc, char *argv[])
long uffd; /* userfaultfd file descriptor */
char *addr; /* Start of region handled by userfaultfd */
unsigned long len; /* Length of region handled by userfaultfd */
pthread_t thr; /* ID of thread that handles page faults */
struct uffdio_api uffdio_api;
struct uffdio_register uffdio_register;
int s;

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

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

/* Create and enable userfaultfd object */

uffd = syscall(__NR_userfaultfd, O_CLOEXEC | O_NONBLOCK);
if (uffd == -1)

uffdio_api.api = UFFD_API;
uffdio_api.features = 0;
if (ioctl(uffd, UFFDIO_API, &uffdio_api) == -1)

/* Create a private anonymous mapping. The memory will be
demand-zero paged--that is, not yet allocated. When we
actually touch the memory, it will be allocated via
the userfaultfd. */

addr = mmap(NULL, len, PROT_READ | PROT_WRITE,
if (addr == MAP_FAILED)

printf("Address returned by mmap() = %p\n", addr);

/* Register the memory range of the mapping we just created for
handling by the userfaultfd object. In mode, we request to track
missing pages (i.e., pages that have not yet been faulted in). */

uffdio_register.range.start = (unsigned long) addr;
uffdio_register.range.len = len;
uffdio_register.mode = UFFDIO_REGISTER_MODE_MISSING;
if (ioctl(uffd, UFFDIO_REGISTER, &uffdio_register) == -1)

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

s = pthread_create(&thr, NULL, fault_handler_thread, (void *) uffd);
if (s != 0) {
errno = s;

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

int l;
l = 0xf; /* Ensure that faulting address is not on a page
boundary, in order to test that we correctly
handle that case in fault_handling_thread() */
while (l < len) {
char c = addr[l];
printf("Read address %p in main(): ", addr + l);
printf("%c\n", c);
l += 1024;
usleep(100000); /* Slow things down a little */


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

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

Michael Kerrisk
Linux man-pages maintainer;
Linux/UNIX System Programming Training:
[unhandled content-type:application/x-troff-man]
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