--- name: "userfaultfd" section: "2" platform: "linux" package: "man-pages" version: "6.19-1" release: "Arch Linux 2026-11-01" platforms: ["linux"] url: "https://new.carta.tech/userfaultfd" --- # userfaultfd(2) create a file descriptor for handling page faults in user space ## NAME userfaultfd - create a file descriptor for handling page faults in user space ## LIBRARY Standard C library (*libc*, *-lc*) ## SYNOPSIS ``` #include /* Definition of O_* constants */ #include /* Definition of SYS_* constants */ #include /* Definition of UFFD_* constants */ #include ``` **int syscall(SYS\_userfaultfd, int** *flags***);** *Note*: glibc provides no wrapper for **userfaultfd**(), necessitating the use of [**syscall**(2)](/syscall/2.md). ## DESCRIPTION **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)](/ioctl/2.md). Once the userfaultfd object is configured, the application can use [**read**(2)](/read/2.md) 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)](/fcntl/2.md). 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)](/open/2.md). - **O\_NONBLOCK** Enables non-blocking operation for the userfaultfd object. See the description of the **O\_NONBLOCK** flag in [**open**(2)](/open/2.md). - **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. - **UFFDIO\_REGISTER\_MODE\_RWP** (since Linux 7.2) When registered with **UFFDIO\_REGISTER\_MODE\_RWP** mode, user space will receive a page-fault notification on any access —read or write— to a page present within the range. By default, the faulted thread will be stopped from execution until user space removes the protection using a **UFFDIO\_RWPROTECT** ioctl; if **UFFD\_FEATURE\_RWP\_ASYNC** was negotiated, the kernel restores access in place and the faulted thread continues without blocking. - **UFFDIO\_REGISTER\_MODE\_RWP** and **UFFDIO\_REGISTER\_MODE\_WP** cannot be combined on the same range; attempting to register with both bits set fails with **EINVAL**. See the "Userfaultfd read-write-protect mode" section below. 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)](/ioctl_userfaultfd/2.md). 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)](/fork/2.md), 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)](/read/2.md). 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. Since Linux 7.2, userfaultfd can do read-write-protection tracking, which traps every access (read or write) to a page present within a registered range. One should check against the feature bit **UFFD\_FEATURE\_RWP** before using this feature, and optionally negotiate **UFFD\_FEATURE\_RWP\_ASYNC** to have the kernel auto-restore page permissions on fault without delivering a notification. This mode is intended for working-set tracking by VM memory managers and similar callers; cold pages can then be evicted using independent kernel interfaces. For more information, please refer to the "Userfaultfd read-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)](/ioctl/2.md) 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)](/ioctl/2.md) 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)](/ioctl/2.md) 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)](/ioctl/2.md) 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)](/ioctl/2.md), 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)](/fork/2.md) if used in combination with **UFFD\_FEATURE\_FORK**. Details of the various [**ioctl**(2)](/ioctl/2.md) operations can be found in [**ioctl\_userfaultfd**(2)](/ioctl_userfaultfd/2.md). 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. ### Userfaultfd read-write-protect mode (since Linux 7.2) Since Linux 7.2, userfaultfd supports read-write-protect mode. Unlike write-protect mode, every access —read or write— to a protected page generates a userfaultfd notification. It works on anonymous, shmem, and hugetlbfs mappings. The user needs to first check availability of this feature using the **UFFDIO\_API** ioctl against the feature bit **UFFD\_FEATURE\_RWP** before using this mode. See [**UFFDIO\_API**(2const)](/UFFDIO_API/2const.md) for the recommended discovery sequence. To register with userfaultfd read-write-protect mode, the user needs to initiate the **UFFDIO\_REGISTER** ioctl with mode **UFFDIO\_REGISTER\_MODE\_RWP** set. **UFFDIO\_REGISTER\_MODE\_RWP** cannot be combined with **UFFDIO\_REGISTER\_MODE\_WP**; however it can be combined with **UFFDIO\_REGISTER\_MODE\_MISSING** when the caller also wants notifications for fresh page populations. After registration, the user can read-write-protect any existing memory within the range using the **UFFDIO\_RWPROTECT** ioctl where *uffdio\_rwprotect.mode* is set to **UFFDIO\_RWPROTECT\_MODE\_RWP**. Read-write protection only affects pages that are currently populated in the range; unpopulated addresses remain unpopulated and fall through to the normal missing-page path on first access. For anonymous mappings, protection is preserved across page reclaim (the marker rides on the swap entry) and migration. For shmem and file-backed mappings, protection is dropped when the backing page is reclaimed and must be re-armed by the caller. Protection is also *not* preserved across operations that explicitly drop the underlying page: **MADV\_DONTNEED** on anonymous memory, hole-punch on shmem, truncation of a file mapping. Callers must re-arm the range with **UFFDIO\_RWPROTECT** after any such operation. When an access fault happens against a protected page, user space will receive a page-fault notification whose *uffd\_msg.pagefault.flags* field has the **UFFD\_PAGEFAULT\_FLAG\_RWP** bit set. To resolve a read-write-protect page fault, the user initiates another **UFFDIO\_RWPROTECT** ioctl whose *uffdio\_rwprotect.mode* has the **UFFDIO\_RWPROTECT\_MODE\_RWP** flag cleared. This restores the original VMA permissions on the affected pages and wakes any blocked threads (unless **UFFDIO\_RWPROTECT\_MODE\_DONTWAKE** is also set). If **UFFD\_FEATURE\_RWP\_ASYNC** was negotiated alongside **UFFD\_FEATURE\_RWP**, the kernel resolves access faults in place without delivering a notification: page permissions are restored automatically and the faulting thread continues. Callers can later reconstruct which pages were touched by inspecting the **PAGE\_IS\_ACCESSED** bit returned by the **PAGEMAP\_SCAN** ioctl described in [**ioctl\_userfaultfd**(2)](/ioctl_userfaultfd/2.md) and *Documentation/admin-guide/mm/pagemap.rst* in the Linux kernel source. The async mode can be toggled at runtime using the **UFFDIO\_SET\_MODE** ioctl, which lets a single userfaultfd switch between async detection and synchronous eviction without re-registering the range. ### Reading from the userfaultfd structure Each [**read**(2)](/read/2.md) 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 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)](/read/2.md) returns as many events as will fit in the supplied buffer. If the buffer supplied to [**read**(2)](/read/2.md) is smaller than the size of the *uffd\_msg* structure, the [**read**(2)](/read/2.md) 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)](/ioctl/2.md). - 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)](/fork/2.md) (or [**clone**(2)](/clone/2.md) 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)](/mremap/2.md). The event details are available in the *remap* field. - **UFFD\_EVENT\_REMOVE** (since Linux 4.11) Generated when the faulting process invokes [**madvise**(2)](/madvise/2.md) 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)](/munmap/2.md) or implicitly during [**mmap**(2)](/mmap/2.md) or [**mremap**(2)](/mremap/2.md). 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\_RWP** (since Linux 7.2) If this flag is set, then the fault was a read-write-protect fault. - **UFFD\_PAGEFAULT\_FLAG\_WRITE** If this flag is set, then the fault was a write fault. If neither **UFFD\_PAGEFAULT\_FLAG\_WP**, **UFFD\_PAGEFAULT\_FLAG\_MINOR**, nor **UFFD\_PAGEFAULT\_FLAG\_RWP** 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)](/fork/2.md). - *remap.from* The original address of the memory range that was remapped using [**mremap**(2)](/mremap/2.md). - *remap.to* The new address of the memory range that was remapped using [**mremap**(2)](/mremap/2.md). - *remap.len* The original size of the memory range that was remapped using [**mremap**(2)](/mremap/2.md). - *remove.start* The start address of the memory range that was freed using [**madvise**(2)](/madvise/2.md) or unmapped - *remove.end* The end address of the memory range that was freed using [**madvise**(2)](/madvise/2.md) or unmapped A [**read**(2)](/read/2.md) 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)](/ioctl/2.md) operation If the **O\_NONBLOCK** flag is enabled in the associated open file description, the userfaultfd file descriptor can be monitored with [**poll**(2)](/poll/2.md), [**select**(2)](/select/2.md), and [**epoll**(7)](/epoll/7.md). When events are available, the file descriptor indicates as readable. If the **O\_NONBLOCK** flag is not enabled, then [**poll**(2)](/poll/2.md) (always) indicates the file as having a **POLLERR** condition, and [**select**(2)](/select/2.md) 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 to indicate the error. ## 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/unprivileged\_userfaultfd* has the value 0. ## STANDARDS Linux. ## HISTORY Linux 4.3. Support for hugetlbfs and shared memory areas and non-page-fault events was added in Linux 4.11 Read-write-protect mode (**UFFDIO\_REGISTER\_MODE\_RWP**, **UFFD\_FEATURE\_RWP**, **UFFDIO\_RWPROTECT**) was added in Linux 7.2, together with **UFFD\_FEATURE\_RWP\_ASYNC** and the **UFFDIO\_SET\_MODE** runtime mode toggle. ## NOTES 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)](/mmap/2.md). 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 If the **UFFD\_FEATURE\_EVENT\_FORK** is enabled and a system call from the [**fork**(2)](/fork/2.md) 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 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)](/mmap/2.md). 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)](/ioctl/2.md) 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)](/poll/2.md) 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)](/ioctl/2.md) 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 #include #include #include #include #include #include #include #include #include #include #include #include #include static int page_size; static void * fault_handler_thread(void *arg) { long uffd; /* userfaultfd file descriptor */ 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 (;;) { int nready; ssize_t nread; struct pollfd pollfd; struct uffdio_copy uffdio_copy; /* 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 = %w64x; ", msg.arg.pagefault.flags); printf("address = %w64x\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 %w64d)\n", uffdio_copy.copy); } } int main(int argc, char *argv[]) { int s; 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) { char c; 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 [**fcntl**(2)](/fcntl/2.md), [**ioctl**(2)](/ioctl/2.md), [**ioctl\_userfaultfd**(2)](/ioctl_userfaultfd/2.md), [**madvise**(2)](/madvise/2.md), [**mmap**(2)](/mmap/2.md) *Documentation/admin-guide/mm/userfaultfd.rst* in the Linux kernel source tree