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 re-
turns 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 de-
scriptor. 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 un-
registered 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 de-
scriptor. The fault-handling thread reads events from this file de-
scriptor 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 de-
termine 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_ZEROPAGE 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 de-
livered 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 fea-
ture 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 buf-
fer. 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 de-
tails 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 ap-
propriately.
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/un-
privileged_userfaultfd has the value 0.
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 in-
tended 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.
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 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 ac-
cessed, 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/admin-guide/mm/userfaultfd.rst in the Linux kernel source
tree
COLOPHON
This page is part of release 5.07 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 2020-06-09 USERFAULTFD(2)