Section (2) ioctl_userfaultfd
Name
ioctl_userfaultfd — create a file descriptor for handling page faults in user space
Synopsis
#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 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.
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
orMADV_REMOVE
advice value to free a virtual memory area the userfaultfd monitor will receive an event of typeUFFD_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 theMAP_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, aSIGBUS
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_zsingle_quotesz_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 thefeatures
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_zsingle_quotesz_s accessible address space. - EINVAL
-
An invalid or unsupported bit was specified in the
mode
field; or themode
field was zero. - EINVAL
-
There is no mapping in the specified address range.
- EINVAL
-
range.start
orrange.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 thelen
field of theufdio_range
structure was not a multiple of the system page size; or thelen
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_zsingle_quotesz_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 thelen
field. - EINVAL
-
Either
dst
orlen
was not a multiple of the system page size, or the range specified bysrc
andlen
ordst
andlen
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
returned in the zeropage
field doesn_zsingle_quotesz_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 therange.len
field. - EINVAL
-
Either
range.start
orrange.len
was not a multiple of the system page size; orrange.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_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 thelen
field of theufdio_range
structure was not a multiple of the system page size; orlen
was zero; or the specified range was otherwise invalid.
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 theUFFDIO_API
operation).
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.
SEE ALSO
ioctl(2), mmap(2), userfaultfd(2)
Documentation/admin-guide/mm/userfaultfd.rst
in the Linux kernel source tree
COLOPHON
This page is part of release 5.04 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/.
Copyright (c) 2016, IBM Corporation. Written by Mike Rapoport <rpptlinux.vnet.ibm.com> and Copyright (C) 2016 Michael Kerrisk <mtk.manpagesgmail.com> %%%LICENSE_START(VERBATIM) Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Since the Linux kernel and libraries are constantly changing, this manual page may be incorrect or out-of-date. The author(s) assume no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein. The author(s) may not have taken the same level of care in the production of this manual, which is licensed free of charge, as they might when working professionally. Formatted or processed versions of this manual, if unaccompanied by the source, must acknowledge the copyright and authors of this work. %%%LICENSE_END |