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SubjectReview request: draft ioctl_userfaultfd(2) manual page
From
Date
Hello Andrea, Mike, and all,

Mike: here's the split out page that describes the
userfaultfd ioctl() operations.

I'd like to get review input, especially from you and
Andrea, but also anyone else, for the current version
of this page, which includes quite a few FIXMEs to be
sorted.

I've shown the rendered version of the page below.
The groff source is attached, and can also be found
at the branch here:

https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/log/?h=draft_userfaultfd

The new ioctl_userfaultfd(2) page follows this mail.

Cheers,

Michael

NAME
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
in a consistent way.


┌─────────────────────────────────────────────────────┐
│FIXME │
├─────────────────────────────────────────────────────┤
│Above: What does "consistent" mean? │
│ │
└─────────────────────────────────────────────────────┘

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; /* Must be zero */
__u64 ioctls; /* Available ioctl() operations (output) */
};

The api field denotes the API version requested by the application.
Before the call, the features field must be initialized to zero.


┌─────────────────────────────────────────────────────┐
│FIXME │
├─────────────────────────────────────────────────────┤
│Above: Why must the 'features' field be initialized │
│to zero? │
└─────────────────────────────────────────────────────┘

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. Cur‐
rently, zero (i.e., no feature bits) is placed in the features field.
The returned 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 below? │
└─────────────────────────────────────────────────────┘

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.


┌─────────────────────────────────────────────────────┐
│FIXME │
├─────────────────────────────────────────────────────┤
│Is the above description of the 'ioctls' field cor‐ │
│rect? Does more need to be said? │
│ │
└─────────────────────────────────────────────────────┘

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:


┌─────────────────────────────────────────────────────┐
│FIXME │
├─────────────────────────────────────────────────────┤
│Is the following error list correct? │
│ │
└─────────────────────────────────────────────────────┘

EINVAL The userfaultfd has already been enabled by a previous UFF‐
DIO_API operation.

EINVAL The API version requested in the api field is not supported by
this kernel, or the features field was not zero.


┌─────────────────────────────────────────────────────┐
│FIXME │
├─────────────────────────────────────────────────────┤
│In the above error case, the returned 'uffdio_api' │
│structure zeroed out. Why is this done? This should │
│be explained in the manual page. │
│ │
└─────────────────────────────────────────────────────┘

UFFDIO_REGISTER
(Since Linux 4.3.) Register a memory address range with the user‐
faultfd object. 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 rnage (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 user‐
faultfd 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 spec‐
ified 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:


┌─────────────────────────────────────────────────────┐
│FIXME │
├─────────────────────────────────────────────────────┤
│Is the following error list correct? │
│ │
└─────────────────────────────────────────────────────┘

EBUSY A mapping in the specified range is registered with another
userfaultfd object.

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.


┌─────────────────────────────────────────────────────┐
│FIXME │
├─────────────────────────────────────────────────────┤
│Above: What does "incompatible" mean? │
│ │
└─────────────────────────────────────────────────────┘

UFFDIO_UNREGISTER
(Since Linux 4.3.) Unregister a memory address range from userfaultfd.
The address range to unregister is specified in the uffdio_range struc‐
ture 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.


┌─────────────────────────────────────────────────────┐
│FIXME │
├─────────────────────────────────────────────────────┤
│Above: What does "incompatible" mean? │
└─────────────────────────────────────────────────────┘

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 struc‐
ture pointed to by argp:

struct uffdio_copy {
__u64 dst; /* Source of copy */
__u64 src; /* Destinate 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).


┌─────────────────────────────────────────────────────┐
│FIXME │
├─────────────────────────────────────────────────────┤
│Above: Why is the 'copy' field used to return error │
│values? This should be explained in the manual │
│page. │
└─────────────────────────────────────────────────────┘
If the value returned in copy doesn't match the value that was speci‐
fied 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.

UFFDIO_ZEROPAGE
(Since Linux 4.3.) Zero out a memory range registered with user‐
faultfd. 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 UFF‐
DIO_COPY.


┌─────────────────────────────────────────────────────┐
│FIXME │
├─────────────────────────────────────────────────────┤
│Why is the 'zeropage' field used to return error │
│values? This should be explained in the manual │
│page. │
└─────────────────────────────────────────────────────┘
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 UFF‐
DIO_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 resolu‐
tion on a specified memory address range. The argp argument is a
pointer to a uffdio_range structure (shown above) that specifies the
address range.


┌─────────────────────────────────────────────────────┐
│FIXME │
├─────────────────────────────────────────────────────┤
│Need more detail here. How is the UFFDIO_WAKE opera‐ │
│tion used? │
└─────────────────────────────────────────────────────┘

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.

RETURN VALUE
See descriptions of the individual operations, above.

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 the UFFDIO_API operation).

CONFORMING TO
These ioctl(2) operations are Linux-specific.

EXAMPLE
See userfaultfd(2).

SEE ALSO
ioctl(2), mmap(2), userfaultfd(2)

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

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