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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.

       Since Linux 4.11, the following feature bits may be set:

       UFFD_FEATURE_EVENT_FORK
              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
              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
              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
              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
              If  this  feature bit is set, the kernel supports registering userfaultfd ranges on
              hugetlbfs virtual memory areas

       UFFD_FEATURE_MISSING_SHMEM
              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.

              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 (since Linux 4.11 till 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 4.13 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/.