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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 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 (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  the  MAP_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, a  SIGBUS  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'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 (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_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.

       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 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.

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.

EXAMPLE

       See userfaultfd(2).

SEE ALSO

       ioctl(2), mmap(2), userfaultfd(2)

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

COLOPHON

       This  page  is  part of release 4.16 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/.