ioctl_userfaultfd - create a file descriptor for handling page faults in user
space
#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:
- 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: 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.
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 cause of 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.
(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 specified range:
- UFFDIO_REGISTER_MODE_MISSING
- Track page faults on missing pages.
- UFFDIO_REGISTER_MODE_WP
- Track page faults on write-protected pages.
Currently, the only supported mode is
UFFDIO_REGISTER_MODE_MISSING.
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 cause of 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.
(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 cause of 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.
(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:
- UFFDIO_COPY_MODE_DONTWAKE
- Do not wake up the thread that waits for page-fault resolution
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 cause of 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 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.
(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 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 cause of the error. Possible 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.
(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 cause of 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.
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:
- 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).
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