IOCTL_USERFAULTFD(2) System Calls Manual IOCTL_USERFAULTFD(2)

ioctl_userfaultfd - create a file descriptor for handling page faults in user space

Standard C library (libc, -lc)

#include <linux/userfaultfd.h>  /* Definition of UFFD* constants */
#include <sys/ioctl.h>
int ioctl(int fd, int cmd, ...);

Various ioctl(2) operations can be performed on a userfaultfd object (created by a call to userfaultfd(2)) using calls of the form:


ioctl(fd, cmd, argp);
In the above, fd is a file descriptor referring to a userfaultfd object, cmd is one of the commands listed below, and argp is a pointer to a data structure that is specific to cmd.

The various ioctl(2) operations are described below. The UFFDIO_API, UFFDIO_REGISTER, and UFFDIO_UNREGISTER operations are used to configure userfaultfd behavior. These operations allow the caller to choose what features will be enabled and what kinds of events will be delivered to the application. The remaining operations are range operations. These operations enable the calling application to resolve page-fault events.

(Since Linux 4.3.) Enable operation of the userfaultfd and perform API handshake.

The argp argument is a pointer to a uffdio_api structure, defined as:


struct uffdio_api {

__u64 api; /* Requested API version (input) */
__u64 features; /* Requested features (input/output) */
__u64 ioctls; /* Available ioctl() operations (output) */ };

The api field denotes the API version requested by the application.

The kernel verifies that it can support the requested API version, and sets the features and ioctls fields to bit masks representing all the available features and the generic ioctl(2) operations available.

For Linux kernel versions before 4.11, the features field must be initialized to zero before the call to UFFDIO_API, and zero (i.e., no feature bits) is placed in the features field by the kernel upon return from ioctl(2).

Starting from Linux 4.11, the features field can be used to ask whether particular features are supported and explicitly enable userfaultfd features that are disabled by default. The kernel always reports all the available features in the features field.

To enable userfaultfd features the application should set a bit corresponding to each feature it wants to enable in the features field. If the kernel supports all the requested features it will enable them. Otherwise it will zero out the returned uffdio_api structure and return EINVAL.

The following feature bits may be set:

When this feature is enabled, the userfaultfd objects associated with a parent process are duplicated into the child process during fork(2) and a UFFD_EVENT_FORK event is delivered to the userfaultfd monitor
If this feature is enabled, when the faulting process invokes mremap(2), the userfaultfd monitor will receive an event of type UFFD_EVENT_REMAP.
If this feature is enabled, when the faulting process calls madvise(2) with the MADV_DONTNEED or MADV_REMOVE advice value to free a virtual memory area the userfaultfd monitor will receive an event of type UFFD_EVENT_REMOVE.
If this feature is enabled, when the faulting process unmaps virtual memory either explicitly with munmap(2), or implicitly during either mmap(2) or mremap(2), the userfaultfd monitor will receive an event of type UFFD_EVENT_UNMAP.
If this feature bit is set, the kernel supports registering userfaultfd ranges on hugetlbfs virtual memory areas
If this feature bit is set, the kernel supports registering userfaultfd ranges on shared memory areas. This includes all kernel shared memory APIs: System V shared memory, tmpfs(5), shared mappings of /dev/zero, mmap(2) with the MAP_SHARED flag set, memfd_create(2), and so on.
If this feature bit is set, no page-fault events (UFFD_EVENT_PAGEFAULT) will be delivered. Instead, a SIGBUS signal will be sent to the faulting process. Applications using this feature will not require the use of a userfaultfd monitor for processing memory accesses to the regions registered with userfaultfd.
If this feature bit is set, uffd_msg.pagefault.feat.ptid will be set to the faulted thread ID for each page-fault message.
If this feature bit is set, the kernel supports registering userfaultfd ranges in minor mode on hugetlbfs-backed memory areas.
If this feature bit is set, the kernel supports registering userfaultfd ranges in minor mode on shmem-backed memory areas.

The returned ioctls field can contain the following bits:

1 << _UFFDIO_API
The UFFDIO_API operation is supported.
1 << _UFFDIO_REGISTER
The UFFDIO_REGISTER operation is supported.
1 << _UFFDIO_UNREGISTER
The UFFDIO_UNREGISTER operation is supported.

This ioctl(2) operation returns 0 on success. On error, -1 is returned and errno is set to indicate the error. Possible errors include:

argp refers to an address that is outside the calling process's accessible address space.
The userfaultfd has already been enabled by a previous UFFDIO_API operation.
The API version requested in the api field is not supported by this kernel, or the features field passed to the kernel includes feature bits that are not supported by the current kernel version.

(Since Linux 4.3.) Register a memory address range with the userfaultfd object. The pages in the range must be "compatible". Please refer to the list of register modes below for the compatible memory backends for each mode.

The argp argument is a pointer to a uffdio_register structure, defined as:


struct uffdio_range {

__u64 start; /* Start of range */
__u64 len; /* Length of range (bytes) */ }; struct uffdio_register {
struct uffdio_range range;
__u64 mode; /* Desired mode of operation (input) */
__u64 ioctls; /* Available ioctl() operations (output) */ };

The range field defines a memory range starting at start and continuing for len bytes that should be handled by the userfaultfd.

The mode field defines the mode of operation desired for this memory region. The following values may be bitwise ORed to set the userfaultfd mode for the specified range:

Track page faults on missing pages. Since Linux 4.3, only private anonymous ranges are compatible. Since Linux 4.11, hugetlbfs and shared memory ranges are also compatible.
Track page faults on write-protected pages. Since Linux 5.7, only private anonymous ranges are compatible.
Track minor page faults. Since Linux 5.13, only hugetlbfs ranges are compatible. Since Linux 5.14, compatiblity with shmem ranges was added.

If the operation is successful, the kernel modifies the ioctls bit-mask field to indicate which ioctl(2) operations are available for the specified range. This returned bit mask can contain the following bits:

1 << _UFFDIO_COPY
The UFFDIO_COPY operation is supported.
1 << _UFFDIO_WAKE
The UFFDIO_WAKE operation is supported.
1 << _UFFDIO_WRITEPROTECT
The UFFDIO_WRITEPROTECT
1 << _UFFDIO_ZEROPAGE
The UFFDIO_ZEROPAGE operation is supported.
1 << _UFFDIO_CONTINUE
The UFFDIO_CONTINUE operation is supported.

This ioctl(2) operation returns 0 on success. On error, -1 is returned and errno is set to indicate the error. Possible errors include:

A mapping in the specified range is registered with another userfaultfd object.
argp refers to an address that is outside the calling process's accessible address space.
An invalid or unsupported bit was specified in the mode field; or the mode field was zero.
There is no mapping in the specified address range.
range.start or range.len is not a multiple of the system page size; or, range.len is zero; or these fields are otherwise invalid.
There as an incompatible mapping in the specified address range.

(Since Linux 4.3.) Unregister a memory address range from userfaultfd. The pages in the range must be "compatible" (see the description of UFFDIO_REGISTER.)

The address range to unregister is specified in the uffdio_range structure pointed to by argp.

This ioctl(2) operation returns 0 on success. On error, -1 is returned and errno is set to indicate the error. Possible errors include:

Either the start or the len field of the ufdio_range structure was not a multiple of the system page size; or the len field was zero; or these fields were otherwise invalid.
There as an incompatible mapping in the specified address range.
There was no mapping in the specified address range.

(Since Linux 4.3.) Atomically copy a continuous memory chunk into the userfault registered range and optionally wake up the blocked thread. The source and destination addresses and the number of bytes to copy are specified by the src, dst, and len fields of the uffdio_copy structure pointed to by argp:


struct uffdio_copy {

__u64 dst; /* Destination of copy */
__u64 src; /* Source of copy */
__u64 len; /* Number of bytes to copy */
__u64 mode; /* Flags controlling behavior of copy */
__s64 copy; /* Number of bytes copied, or negated error */ };

The following value may be bitwise ORed in mode to change the behavior of the UFFDIO_COPY operation:

Do not wake up the thread that waits for page-fault resolution
Copy the page with read-only permission. This allows the user to trap the next write to the page, which will block and generate another write-protect userfault message. This is used only when both UFFDIO_REGISTER_MODE_MISSING and UFFDIO_REGISTER_MODE_WP modes are enabled for the registered range.

The copy field is used by the kernel to return the number of bytes that was actually copied, or an error (a negated errno-style value). If the value returned in copy doesn't match the value that was specified in len, the operation fails with the error EAGAIN. The copy field is output-only; it is not read by the UFFDIO_COPY operation.

This ioctl(2) operation returns 0 on success. In this case, the entire area was copied. On error, -1 is returned and errno is set to indicate the error. Possible errors include:

The number of bytes copied (i.e., the value returned in the copy field) does not equal the value that was specified in the len field.
Either dst or len was not a multiple of the system page size, or the range specified by src and len or dst and len was invalid.
An invalid bit was specified in the mode field.
The faulting process has changed its virtual memory layout simultaneously with an outstanding UFFDIO_COPY operation.
The faulting process has exited at the time of a UFFDIO_COPY operation.
The faulting process has exited at the time of a UFFDIO_COPY operation.

(Since Linux 4.3.) Zero out a memory range registered with userfaultfd.

The requested range is specified by the range field of the uffdio_zeropage structure pointed to by argp:


struct uffdio_zeropage {

struct uffdio_range range;
__u64 mode; /* Flags controlling behavior of copy */
__s64 zeropage; /* Number of bytes zeroed, or negated error */ };

The following value may be bitwise ORed in mode to change the behavior of the UFFDIO_ZEROPAGE operation:

Do not wake up the thread that waits for page-fault resolution.

The zeropage field is used by the kernel to return the number of bytes that was actually zeroed, or an error in the same manner as UFFDIO_COPY. If the value returned in the zeropage field doesn't match the value that was specified in range.len, the operation fails with the error EAGAIN. The zeropage field is output-only; it is not read by the UFFDIO_ZEROPAGE operation.

This ioctl(2) operation returns 0 on success. In this case, the entire area was zeroed. On error, -1 is returned and errno is set to indicate the error. Possible errors include:

The number of bytes zeroed (i.e., the value returned in the zeropage field) does not equal the value that was specified in the range.len field.
Either range.start or range.len was not a multiple of the system page size; or range.len was zero; or the range specified was invalid.
An invalid bit was specified in the mode field.
The faulting process has exited at the time of a UFFDIO_ZEROPAGE operation.

(Since Linux 4.3.) Wake up the thread waiting for page-fault resolution on a specified memory address range.

The UFFDIO_WAKE operation is used in conjunction with UFFDIO_COPY and UFFDIO_ZEROPAGE operations that have the UFFDIO_COPY_MODE_DONTWAKE or UFFDIO_ZEROPAGE_MODE_DONTWAKE bit set in the mode field. The userfault monitor can perform several UFFDIO_COPY and UFFDIO_ZEROPAGE operations in a batch and then explicitly wake up the faulting thread using UFFDIO_WAKE.

The argp argument is a pointer to a uffdio_range structure (shown above) that specifies the address range.

This ioctl(2) operation returns 0 on success. On error, -1 is returned and errno is set to indicate the error. Possible errors include:

The start or the len field of the ufdio_range structure was not a multiple of the system page size; or len was zero; or the specified range was otherwise invalid.

Write-protect or write-unprotect a userfaultfd-registered memory range registered with mode UFFDIO_REGISTER_MODE_WP.

The argp argument is a pointer to a uffdio_range structure as shown below:


struct uffdio_writeprotect {

struct uffdio_range range; /* Range to change write permission*/
__u64 mode; /* Mode to change write permission */ };

There are two mode bits that are supported in this structure:

When this mode bit is set, the ioctl will be a write-protect operation upon the memory range specified by range. Otherwise it will be a write-unprotect operation upon the specified range, which can be used to resolve a userfaultfd write-protect page fault.
When this mode bit is set, do not wake up any thread that waits for page-fault resolution after the operation. This can be specified only if UFFDIO_WRITEPROTECT_MODE_WP is not specified.

This ioctl(2) operation returns 0 on success. On error, -1 is returned and errno is set to indicate the error. Possible errors include:

The start or the len field of the ufdio_range structure was not a multiple of the system page size; or len was zero; or the specified range was otherwise invalid.
The process was interrupted; retry this call.
The range specified in range is not valid. For example, the virtual address does not exist, or not registered with userfaultfd write-protect mode.
Encountered a generic fault during processing.

(Since Linux 5.13.) Resolve a minor page fault by installing page table entries for existing pages in the page cache.

The argp argument is a pointer to a uffdio_continue structure as shown below:


struct uffdio_continue {

struct uffdio_range range;
/* Range to install PTEs for and continue */
__u64 mode; /* Flags controlling the behavior of continue */
__s64 mapped; /* Number of bytes mapped, or negated error */ };

The following value may be bitwise ORed in mode to change the behavior of the UFFDIO_CONTINUE operation:

Do not wake up the thread that waits for page-fault resolution.

The mapped field is used by the kernel to return the number of bytes that were actually mapped, or an error in the same manner as UFFDIO_COPY. If the value returned in the mapped field doesn't match the value that was specified in range.len, the operation fails with the error EAGAIN. The mapped field is output-only; it is not read by the UFFDIO_CONTINUE operation.

This ioctl(2) operation returns 0 on success. In this case, the entire area was mapped. On error, -1 is returned and errno is set to indicate the error. Possible errors include:

The number of bytes mapped (i.e., the value returned in the mapped field) does not equal the value that was specified in the range.len field.
Either range.start or range.len was not a multiple of the system page size; or range.len was zero; or the range specified was invalid.
An invalid bit was specified in the mode field.
One or more pages were already mapped in the given range.
The faulting process has changed its virtual memory layout simultaneously with an outstanding UFFDIO_CONTINUE operation.
Allocating memory needed to setup the page table mappings failed.
No existing page could be found in the page cache for the given range.
The faulting process has exited at the time of a UFFDIO_CONTINUE operation.

See descriptions of the individual operations, above.

See descriptions of the individual operations, above. In addition, the following general errors can occur for all of the operations described above:

argp does not point to a valid memory address.
(For all operations except UFFDIO_API.) The userfaultfd object has not yet been enabled (via the UFFDIO_API operation).

These ioctl(2) operations are Linux-specific.

In order to detect available userfault features and enable some subset of those features the userfaultfd file descriptor must be closed after the first UFFDIO_API operation that queries features availability and reopened before the second UFFDIO_API operation that actually enables the desired features.

See userfaultfd(2).

ioctl(2), mmap(2), userfaultfd(2)

Documentation/admin-guide/mm/userfaultfd.rst in the Linux kernel source tree

2022-10-09 Linux man-pages 6.01