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.TH IOCTL_USERFAULTFD 2 2016-12-12 "Linux" "Linux Programmer's Manual"
.SH NAME
userfaultfd \- create a file descriptor for handling page faults in user
space
.SH SYNOPSIS
.nf
.B #include <sys/ioctl.h>

.BI "int ioctl(int " fd ", int " cmd ", ...);"
.fi
.SH DESCRIPTION
Various
.BR ioctl (2)
operations can be performed on a userfaultfd object (created by a call to
.BR userfaultfd (2))
using calls of the form:

    ioctl(fd, cmd, argp);

In the above,
.I fd
is a file descriptor referring to a userfaultfd object,
.I cmd
is one of the commands listed below, and
.I argp
is a pointer to a data structure that is specific to
.IR cmd .

The various
.BR ioctl (2)
operations are described below.
The
.BR UFFDIO_API,
.BR UFFDIO_REGISTER ,
and
.BR UFFDIO_UNREGISTER
operations are used to
.I 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
.IR range
operations.
These operations enable the calling application to resolve page fault
events in a consistent way.
.\" FIXME What does "consistent" mean?
.\"
.SS UFFDIO_API
(Since Linux 4.3.)
Enable operation of the userfaultfd and perform API handshake.
The
.I argp
argument is a pointer to a
.IR uffdio_api
structure, defined as:
.in +4n
.nf

struct uffdio_api {
    __u64 api;
    __u64 features;
    __u64 ioctls;
};

.fi
.in
The
.I api
field denotes the API version requested by the application.
Before the call, the
.I features
field must be initialized to zero.
.\" FIXME Why must the 'features' field be initialized to zero?

The kernel verifies that it can support the requested API version,
and sets the
.I features
and
.I ioctls
fields to bit masks representing all the available features and the generic
.BR ioctl (2)
operations available.
Currently, zero (i.e., no feature bits) is placed in the
.I features
field.
The returned
.I ioctls
field can contain the following bits:
.\" FIXME This user-space API seems not fully polished. Why are there
.\" not constants defined for each of the bit-mask values listed here?
.TP
.B 1 << _UFFDIO_API
The
.B UFFDIO_API
operation is supported.
.TP
.B 1 << _UFFDIO_REGISTER
The
.B UFFDIO_REGISTER
operation is supported.
.TP
.B 1 << _UFFDIO_UNREGISTER
The
.B UFFDIO_UNREGISTER
operation is supported.
.\" FIXME Is the above description of the 'ioctls' field correct.
.\" Does more need to be said?
.\"
.PP
This
.BR ioctl (2)
operation returns 0 on success.
On error, \-1 is returned and
.I errno
is set to indicate the cause of the error.
Possible errors include:
.\" FIXME Is the following error list correct?
.\"
.TP
.B EINVAL
The userfaultfd has already been enabled by a previous
.BR UFFDIO_API
operation.
.TP
.B EINVAL
The API version requested in the
.I api
field is not supported by this kernel, or the
.I features
field was not zero.
.\" FIXME In this error case, the returned 'uffdio_api' structure
.\" zeroed out. Why is this done?
.\"
.SS UFFDIO_REGISTER
(Since Linux 4.3.)
Register a memory address range with the userfaultfd object.
The
.I argp
argument is a pointer to a
.I uffdio_register
structure, defined as:
.in +4n
.nf

struct uffdio_range {
    __u64 start;
    __u64 len;
};

struct uffdio_register {
    struct uffdio_range range;
    __u64 mode;
    __u64 ioctls;
};

.fi
.in

The
.I range
field defines a memory range starting at
.I start
and continuing for
.I len
bytes that should be handled by the userfaultfd.

The
.I 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:
.TP
.B UFFDIO_REGISTER_MODE_MISSING
Track page faults on missing pages.
.TP
.B UFFDIO_REGISTER_MODE_WP
Track page faults on write-protected pages.
.PP
.\" FIXME Actually, the following sentence appears not to be true;
.\" UFFDIO_REGISTER_MODE_WP seems also to be supported.
.\" Can someone confirm?
Currently, the only supported mode is
.BR UFFDIO_REGISTER_MODE_MISSING .
.PP
.\" FIXME In the following, what does "answers" mean, and what are the bits?
.\" (we need a list of the bits here).
The kernel answers which ioctl commands are available for the requested
range in the
.I ioctls
field.
.\"
.SS UFFDIO_UNREGISTER
(Since Linux 4.3.)
Unregister a memory address range from userfaultfd.
The address range to unregister is specified in the
.IR uffdio_range
structure pointed to by
.IR argp .

This
.BR ioctl (2)
operation returns 0 on success.
On error, \-1 is returned and
.I errno
is set to indicate the cause of the error.
Possible errors include:
.TP
.B EINVAL
Either the
.I start
or the
.I len
field of the
.I ufdio_range
structure was not a multiple of the system page size.
.TP
.B EINVAL
There as an incompatible mapping in the specified address range.
.TP
.B EINVAL
There was no mapping in the specified address range.
.\"
.SS 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
.IR src ", " dst ", and " len
fields of the
.I uffdio_copy
structure pointed to by
.IR argp :

.in +4n
.nf
struct uffdio_copy {
    __u64 dst;
    __u64 src;
    __u64 len;
    __u64 mode;
    __s64 copy;
};
.fi
.in
.PP
The following values may be bitwise ORed in
.IR mode
to change the behavior of the
.B UFFDIO_COPY
operation:
.TP
.B UFFDIO_COPY_MODE_DONTWAKE
Do not wake up the thread that waits for page fault resolution
.PP
The
.I copy
field of the
.I uffdio_copy
structure is used by the kernel to return the number of bytes
that was actually copied, or an error.
If
.I uffdio_copy.copy
doesn't match the
.I uffdio_copy.len
passed in input to
.BR UFFDIO_COPY ,
the operation will return
.\" FIXME In the 'copy' field? (This isn't clear.)
.BR \-EAGAIN .
If
.BR ioctl (2)
returns zero it means it succeeded, no error was reported and
the entire area was copied.
If an invalid fault happens while writing to the
.I uffdio_copy.copy
field, the system call will return
.\" FIXME In the 'copy' field? (This isn't clear.)
.BR \-EFAULT .
.I uffdio_copy.copy
is an output-only field;
it is not read by the
.B UFFDIO_COPY
operation.
.\"
.SS UFFDIO_ZERO
(Since Linux 4.3.)
Zero out a part of memory range registered with userfaultfd.
The requested range is specified by the
.I range
field of the
.I uffdio_zeropage
structure pointed to by
.IR argp :

.in +4n
.nf
struct uffdio_zeropage {
    struct uffdio_range range;
    __u64 mode;
    __s64 zeropage;
};
.fi
.in
.PP
The following values may be bitwise ORed in
.IR mode
to change the behavior of the
.B UFFDIO_ZERO
operation:
.TP
.B UFFDIO_ZEROPAGE_MODE_DONTWAKE
Do not wake up the thread that waits for page-fault resolution.
.PP
The
.I zeropage
field of the
.I uffdio_zero
structure is used by the kernel to return the number of bytes
that was actually zeroed,
or an error in the same manner as
.IR uffdio_copy.copy .
.\"
.SS UFFDIO_WAKE
(Since Linux 4.3.)
Wake up the thread waiting for page-fault resolution.
The
.I argp
argument is a pointer to a
.I uffdio_range
structure (shown above).
.\" FIXME: Need more detail here. What is the purpose of the
.\" 'struct uffdio_range *' argument? 

This
.BR ioctl (2)
operation returns 0 on success.
On error, \-1 is returned and
.I errno
is set to indicate the cause of the error.
Possible errors include:
.TP
.B EINVAL
Either the
.I start
or the
.I len
field of the
.I ufdio_range
structure was not a multiple of the system page size.
.SH RETURN VALUE
See descriptions of the individual operations, above.
.SH ERRORS
See descriptions of the individual operations, above.
In addition, the following general errors can occur for all of the
operations described above:
.TP
.B EFAULT
.I argp
does not point to a valid memory address.
.TP
.B EINVAL
(For all operations except
.BR UFFDIO_API .)
The userfaultfd object has not yet been enabled (via the
.BR UFFDIO_API
operation).
.SH CONFORMING TO
These
.BR ioctl (2)
operations are Linux-specific.
.SH SEE ALSO
.BR ioctl (2),
.BR mmap (2),
.BR userfaultfd (2)

.IR Documentation/vm/userfaultfd.txt
in the Linux kernel source tree