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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | HISTORY | NOTES | BUGS | EXAMPLES | SEE ALSO |
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userfaultfd(2) System Calls Manual userfaultfd(2)
userfaultfd - create a file descriptor for handling page faults
in user space
Standard C library (libc, -lc)
#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).
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 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 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.
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.
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.
Linux.
Linux 4.3.
Support for hugetlbfs and shared memory areas and non-page-fault
events was added in Linux 4.11
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.
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.
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");
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);
}
fcntl(2), ioctl(2), ioctl_userfaultfd(2), madvise(2), mmap(2)
Documentation/admin-guide/mm/userfaultfd.rst in the Linux kernel
source tree
Linux man-pages (unreleased) (date) userfaultfd(2)
Pages that refer to this page: ioctl_userfaultfd(2), mmap(2), mremap(2), syscalls(2), proc(5)
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