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IOCTL_USERFAULTFD(2) Linux Programmer's Manual IOCTL_USERFAULTFD(2)
NAME
ioctl_userfaultfd - create a file descriptor for handling page faults in user space
SYNOPSIS
#include <linux/userfaultfd.h> /* Definition of UFFD* constants */
#include <sys/ioctl.h>
int ioctl(int fd, int cmd, ...);
DESCRIPTION
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 behavâ
ior. 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 opâ
erations are range operations. These operations enable the calling application to resolve page-fault events.
UFFDIO_API
(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:
UFFD_FEATURE_EVENT_FORK (since Linux 4.11)
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
UFFD_FEATURE_EVENT_REMAP (since Linux 4.11)
If this feature is enabled, when the faulting process invokes mremap(2), the userfaultfd monitor will receive an event of type UFFD_EVENT_REMAP.
UFFD_FEATURE_EVENT_REMOVE (since Linux 4.11)
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.
UFFD_FEATURE_EVENT_UNMAP (since Linux 4.11)
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.
UFFD_FEATURE_MISSING_HUGETLBFS (since Linux 4.11)
If this feature bit is set, the kernel supports registering userfaultfd ranges on hugetlbfs virtual memory areas
UFFD_FEATURE_MISSING_SHMEM (since Linux 4.11)
If this feature bit is set, the kernel supports registering userfaultfd ranges on shared memory areas. This includes all kernel shared memory APIs: Sysâ
tem V shared memory, tmpfs(5), shared mappings of /dev/zero, mmap(2) with the MAP_SHARED flag set, memfd_create(2), and so on.
UFFD_FEATURE_SIGBUS (since Linux 4.14)
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 userâ
faultfd.
UFFD_FEATURE_THREAD_ID (since Linux 4.14)
If this feature bit is set, uffd_msg.pagefault.feat.ptid will be set to the faulted thread ID for each page-fault message.
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.
1 << _UFFDIO_WRITEPROTECT
The UFFDIO_WRITEPROTECT 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:
EFAULT argp refers to an address that is outside the calling process's accessible address space.
EINVAL The userfaultfd has already been enabled by a previous UFFDIO_API operation.
EINVAL 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.
UFFDIO_REGISTER
(Since Linux 4.3.) Register a memory address range with the userfaultfd object. The pages in the range must be "compatible".
Up to Linux kernel 4.11, only private anonymous ranges are compatible for registering with UFFDIO_REGISTER.
Since Linux 4.11, hugetlbfs and shared memory ranges are also compatible with UFFDIO_REGISTER.
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 speciâ
fied range:
UFFDIO_REGISTER_MODE_MISSING
Track page faults on missing pages.
UFFDIO_REGISTER_MODE_WP
Track page faults on write-protected pages.
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 is as for UFFDIO_API.
This ioctl(2) operation returns 0 on success. On error, -1 is returned and errno is set to indicate the error. Possible errors include:
EBUSY A mapping in the specified range is registered with another userfaultfd object.
EFAULT argp refers to an address that is outside the calling process's accessible address space.
EINVAL An invalid or unsupported bit was specified in the mode field; or the mode field was zero.
EINVAL There is no mapping in the specified address range.
EINVAL 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.
EINVAL There as an incompatible mapping in the specified address range.
UFFDIO_UNREGISTER
(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:
EINVAL 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.
EINVAL There as an incompatible mapping in the specified address range.
EINVAL There was no mapping in the specified address range.
UFFDIO_COPY
(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 desâ
tination 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:
UFFDIO_COPY_MODE_DONTWAKE
Do not wake up the thread that waits for page-fault resolution
UFFDIO_COPY_MODE_WP
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 UFFâ
DIO_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. Posâ
sible errors include:
EAGAIN 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.
EINVAL 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.
EINVAL An invalid bit was specified in the mode field.
ENOENT (since Linux 4.11)
The faulting process has changed its virtual memory layout simultaneously with an outstanding UFFDIO_COPY operation.
ENOSPC (from Linux 4.11 until Linux 4.13)
The faulting process has exited at the time of a UFFDIO_COPY operation.
ESRCH (since Linux 4.13)
The faulting process has exited at the time of a UFFDIO_COPY operation.
UFFDIO_ZEROPAGE
(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:
UFFDIO_ZEROPAGE_MODE_DONTWAKE
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 reâ
turned 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. Posâ
sible errors include:
EAGAIN 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.
EINVAL 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.
EINVAL An invalid bit was specified in the mode field.
ESRCH (since Linux 4.13)
The faulting process has exited at the time of a UFFDIO_ZEROPAGE operation.
UFFDIO_WAKE
(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_ZEâ
ROPAGE_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 explicâ
itly 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:
EINVAL 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.
UFFDIO_WRITEPROTECT (Since Linux 5.7)
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:
UFFDIO_WRITEPROTECT_MODE_WP
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.
UFFDIO_WRITEPROTECT_MODE_DONTWAKE
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 UFFâ
DIO_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:
EINVAL 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.
EAGAIN The process was interrupted; retry this call.
ENOENT The range specified in range is not valid. For example, the virtual address does not exist, or not registered with userfaultfd write-protect mode.
EFAULT Encountered a generic fault during processing.
RETURN VALUE
See descriptions of the individual operations, above.
ERRORS
See descriptions of the individual operations, above. In addition, the following general errors can occur for all of the operations described above:
EFAULT argp does not point to a valid memory address.
EINVAL (For all operations except UFFDIO_API.) The userfaultfd object has not yet been enabled (via the UFFDIO_API operation).
CONFORMING TO
These ioctl(2) operations are Linux-specific.
BUGS
In order to detect available userfault features and enable some subset of those features the userfaultfd file descriptor must be closed after the first UFFâ
DIO_API operation that queries features availability and reopened before the second UFFDIO_API operation that actually enables the desired features.
EXAMPLES
See userfaultfd(2).
SEE ALSO
ioctl(2), mmap(2), userfaultfd(2)
Documentation/admin-guide/mm/userfaultfd.rst in the Linux kernel source tree
Linux 2021-03-22 IOCTL_USERFAULTFD(2)