ioctl_userfaultfd

IOCTL_USERFAULTFD(2)       Linux Programmer's Manual      IOCTL_USERFAULTFD(2)

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 ini-
       tialized to zero before the call to UFFDIO_API, and zero (i.e., no fea-
       ture  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 corre-
       sponding 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 dur-
              ing  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 mad-
              vise(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 UFF-
              DIO_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  ver-
              sion.

   UFFDIO_REGISTER
       (Since  Linux  4.3.)   Register  a  memory address range with the user-
       faultfd 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 compati-
       ble 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 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:

       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 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.

       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;    /* 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 out-
       put-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 user-
       faultfd.

       The requested range is  specified  by  the  range  field  of  the  uff-
       dio_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.  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  resolu-
       tion 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 UFF-
       DIO_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
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Linux                             2017-09-15              IOCTL_USERFAULTFD(2)
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