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:
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.
The returned ioctls
field can contain the following
bits:
The UFFDIO_API operation is supported.
The UFFDIO_REGISTER operation is supported.
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:
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".
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:
Track page faults on missing pages.
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:
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 cause of
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
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:
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 cause of 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 cause of
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.
See descriptions of the individual operations, above.
See userfaultfd(2).
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.
ioctl(2), mmap(2), userfaultfd(2)
Documentation/admin-guide/mm/userfaultfd.rst
in the Linux
kernel source tree
This page is part of release 5.10 of the Linux man-pages
project. A description of the project, information about reporting bugs,
and the latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.