ioctl_userfaultfd(2) System Calls Manual ioctl_userfaultfd(2)
NAME
ioctl_userfaultfd - create a file descriptor for handling page faults
in user space
LIBRARY
Standard C library (libc, -lc)
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 ob-
ject, 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.
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.
Before Linux 4.11, the features field must be initialized to zero be-
fore 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 corre-
sponding 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 dur-
ing 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 mad-
vise(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: 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.
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
userfaultfd.
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.
UFFD_FEATURE_MINOR_HUGETLBFS (since Linux 5.13)
If this feature bit is set, the kernel supports registering
userfaultfd ranges in minor mode on hugetlbfs-backed memory ar-
eas.
UFFD_FEATURE_MINOR_SHMEM (since Linux 5.14)
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:
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 UFF-
DIO_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 ver-
sion.
UFFDIO_REGISTER
(Since Linux 4.3.) Register a memory address range with the user-
faultfd 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 user-
faultfd mode for the specified range:
UFFDIO_REGISTER_MODE_MISSING
Track page faults on missing pages. Since Linux 4.3, only pri-
vate anonymous ranges are compatible. Since Linux 4.11,
hugetlbfs and shared memory ranges are also compatible.
UFFDIO_REGISTER_MODE_WP
Track page faults on write-protected pages. Since Linux 5.7,
only private anonymous ranges are compatible.
UFFDIO_REGISTER_MODE_MINOR
Track minor page faults. Since Linux 5.13, only hugetlbfs
ranges are compatible. Since Linux 5.14, compatibility 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 spec-
ified 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:
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 in-
valid.
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 struc-
ture 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 destination addresses and the number of bytes to copy
are specified by the src, dst, and len fields of the uffdio_copy struc-
ture 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 gener-
ate another write-protect userfault message. This is used only
when both UFFDIO_REGISTER_MODE_MISSING and UFFDIO_REGIS-
TER_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 out-
put-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:
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 si-
multaneously 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 op-
eration.
ESRCH (since Linux 4.13)
The faulting process has exited at the time of a UFFDIO_COPY op-
eration.
UFFDIO_ZEROPAGE
(Since Linux 4.3.) Zero out a memory range registered with user-
faultfd.
The requested range is specified by the range field of the uffdio_ze-
ropage 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 UFF-
DIO_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:
EAGAIN The number of bytes zeroed (i.e., the value returned in the ze-
ropage 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 in-
valid.
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 resolu-
tion 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 UFF-
DIO_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 be-
low:
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 op-
eration 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 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:
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 user-
faultfd write-protect mode.
EFAULT Encountered a generic fault during processing.
UFFDIO_CONTINUE
(Since Linux 5.13.) Resolve a minor page fault by installing page ta-
ble 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:
UFFDIO_CONTINUE_MODE_DONTWAKE
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 UFF-
DIO_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 er-
ror 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:
EAGAIN 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.
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 in-
valid.
EINVAL An invalid bit was specified in the mode field.
EEXIST One or more pages were already mapped in the given range.
ENOENT The faulting process has changed its virtual memory layout si-
multaneously with an outstanding UFFDIO_CONTINUE operation.
ENOMEM Allocating memory needed to setup the page table mappings
failed.
EFAULT No existing page could be found in the page cache for the given
range.
ESRCH The faulting process has exited at the time of a UFFDIO_CONTINUE
operation.
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).
STANDARDS
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 UFFDIO_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 man-pages 6.03 2022-12-15 ioctl_userfaultfd(2)
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