Review request: draft ioctl_userfaultfd(2) manual page

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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

Attachment: ioctl_userfaultfd.2
Description: Unix manual page


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