userfaultfd — Linux manual page
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https://man7.org/linux/man-pages/man2/userfaultfd.2.html
USERFAULTFD(2) Linux Programmer's Manual USERFAULTFD(2)
NAME top
userfaultfd - create a file descriptor for handling page faults
in user space
SYNOPSIS top
#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 top
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).
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
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.
Starting from 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.
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
or UFFDIO_ZEROPAGE ioctl(2) operations to resolve the page fault.
Starting from 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.
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 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_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.
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.
RETURN VALUE top
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.
ERRORS top
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.
VERSIONS top
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 top
userfaultfd() is Linux-specific and should not be used in
programs intended to be portable.
NOTES top
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 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.
BUGS top
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.
EXAMPLES top
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 <inttypes.h>
#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 = %"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)
errExit("ioctl-UFFDIO_COPY");
printf(" (uffdio_copy.copy returned %"PRId64")\\n",
uffdio_copy.copy);
}
}
int
main(int argc, char *argv[])
{
long uffd; /* userfaultfd file descriptor */
char *addr; /* Start of region handled by userfaultfd */
uint64_t 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 = strtoull(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 top
fcntl(2), ioctl(2), ioctl_userfaultfd(2), madvise(2), mmap(2)
Documentation/admin-guide/mm/userfaultfd.rst in the Linux kernel
source tree
COLOPHON top
This page is part of release 5.11 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 2021-03-22 USERFAULTFD(2)
Pages that refer to this page: ioctl_userfaultfd(2), mmap(2), mremap(2), syscalls(2), proc(5)
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