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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | EXAMPLE | BUGS | SEE ALSO | COLOPHON |
IOCTL_USERFAULTFD(2) Linux Programmer's Manual IOCTL_USERFAULTFD(2)
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.
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; /* Currently must be zero (input) */
__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.
Since Linux 4.11, the following feature bits may be set:
UFFD_FEATURE_EVENT_FORK
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
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
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
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
If this feature bit is set, the kernel supports registering
userfaultfd ranges on hugetlbfs virtual memory areas
UFFD_FEATURE_MISSING_SHMEM
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.
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.
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 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.
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 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.
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 structure pointed to by argp:
struct uffdio_copy {
__u64 dst; /* Source of copy */
__u64 src; /* Destination 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 (since Linux 4.11)
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_ZERO 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_ZERO 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.
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_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.
See userfaultfd(2).
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/vm/userfaultfd.txt in the Linux kernel source tree
This page is part of release 4.12 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/.
Linux 2017-05-03 IOCTL_USERFAULTFD(2)
Pages that refer to this page: ioctl(2), userfaultfd(2)