userfaultfd(2) - Linux manual page (original) (raw)
userfaultfd(2) System Calls Manual userfaultfd(2)
NAME top
userfaultfd - create a file descriptor for handling page faults in
user spaceLIBRARY top
Standard C library (_libc_, _-lc_)SYNOPSIS top
**#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)](../man2/syscall.2.html).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)](../man2/ioctl.2.html).
Once the userfaultfd object is configured, the application can use
[read(2)](../man2/read.2.html) 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)](../man2/fcntl.2.html).
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)](../man2/open.2.html).
**O_NONBLOCK**
Enables non-blocking operation for the userfaultfd object.
See the description of the **O_NONBLOCK** flag in [open(2)](../man2/open.2.html).
**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)](../man2/fork.2.html),
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)](../man2/read.2.html). 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 two-step handshake between the kernel and user space to determine what API version and features the kernel supports, and then to enable those features user space wants. 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 various (e.g., **UFFDIO_COPY**,
**UFFDIO_ZEROPAGE**, or **UFFDIO_CONTINUE**) [ioctl(2)](../man2/ioctl.2.html) 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)](../man2/fork.2.html) if used in combination with **UFFD_FEATURE_FORK**.
Details of the various [ioctl(2)](../man2/ioctl.2.html) operations can be found in
[ioctl_userfaultfd(2)](../man2/ioctl%5Fuserfaultfd.2.html).
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 _uffdiowriteprotect.mode_ should be set
to **UFFDIO_WRITEPROTECT_MODE_WP**.
When a write-protect event happens, user-space will receive a
page-fault notification whose _uffdmsg.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 _uffdmsg.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 _uffdmsg.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 uffdmsg 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 size */
} 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)](../man2/read.2.html) returns as many events as will fit in the supplied
buffer. If the buffer supplied to [read(2)](../man2/read.2.html) is smaller than the
size of the _uffdmsg_ structure, the [read(2)](../man2/read.2.html) fails with the error
**EINVAL**.
The fields set in the _uffdmsg_ 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)](../man2/fork.2.html)
(or [clone(2)](../man2/clone.2.html) 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)](../man2/mremap.2.html). The event details are available in the
_remap_ field.
**UFFD_EVENT_REMOVE** (since Linux 4.11)
Generated when the faulting process invokes
[madvise(2)](../man2/madvise.2.html) 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)](../man2/munmap.2.html) or
implicitly during [mmap(2)](../man2/mmap.2.html) or [mremap(2)](../man2/mremap.2.html). 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)](../man2/fork.2.html).
_remap.from_
The original address of the memory range that was remapped
using [mremap(2)](../man2/mremap.2.html).
_remap.to_
The new address of the memory range that was remapped using
[mremap(2)](../man2/mremap.2.html).
_remap.len_
The original size of the memory range that was remapped
using [mremap(2)](../man2/mremap.2.html).
_remove.start_
The start address of the memory range that was freed using
[madvise(2)](../man2/madvise.2.html) or unmapped
_remove.end_
The end address of the memory range that was freed using
[madvise(2)](../man2/madvise.2.html) or unmapped
A [read(2)](../man2/read.2.html) 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)](../man2/poll.2.html), [select(2)](../man2/select.2.html), and [epoll(7)](../man7/epoll.7.html). When events are available, the
file descriptor indicates as readable. If the **O_NONBLOCK** flag is
not enabled, then [poll(2)](../man2/poll.2.html) (always) indicates the file as having a
**POLLERR** condition, and [select(2)](../man2/select.2.html) 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](../man3/errno.3.html)_ 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/unprivilegeduserfaultfd_ has the value 0.STANDARDS top
Linux.HISTORY top
Linux 4.3.
Support for hugetlbfs and shared memory areas and non-page-fault
events was added in Linux 4.11NOTES top
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)](../man2/mmap.2.html). 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)](../man2/fork.2.html) 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)](../man2/mmap.2.html).
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)](../man2/poll.2.html) 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)](../man2/ioctl.2.html) 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(): CProgram 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 size, i; /* Size 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);
size = 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");
/* NOTE: Two-step feature handshake is not needed here, since this
example doesn't require any specific features.
Programs that *do* should call UFFDIO_API twice: once with
`features = 0` to detect features supported by this kernel, and
again with the subset of features the program actually wants to
enable. */
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, size, 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 = size;
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. */
i = 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 (i < size) {
c = addr[i];
printf("Read address %p in %s(): ", addr + i, __func__);
printf("%c\n", c);
i += 1024;
usleep(100000); /* Slow things down a little */
}
exit(EXIT_SUCCESS);
}SEE ALSO top
[fcntl(2)](../man2/fcntl.2.html), [ioctl(2)](../man2/ioctl.2.html), [ioctl_userfaultfd(2)](../man2/ioctl%5Fuserfaultfd.2.html), [madvise(2)](../man2/madvise.2.html), [mmap(2)](../man2/mmap.2.html)
_Documentation/admin-guide/mm/userfaultfd.rst_ in the Linux kernel
source treeCOLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2024-11-17 userfaultfd(2)
Pages that refer to this page:ioctl_userfaultfd(2), mmap(2), mremap(2), syscalls(2), UFFDIO_API(2const), UFFDIO_CONTINUE(2const), UFFDIO_COPY(2const), UFFDIO_POISON(2const), UFFDIO_REGISTER(2const), UFFDIO_UNREGISTER(2const), UFFDIO_WAKE(2const), UFFDIO_WRITEPROTECT(2const), UFFDIO_ZEROPAGE(2const), proc_pid_fd(5), proc_pid_pagemap(5), proc_sys_vm(5)