ioctl_userfaultfd(2) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | BUGS | EXAMPLES | SEE ALSO | COLOPHON

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

NAME         top

       ioctl_userfaultfd - create a file descriptor for handling page
       faults in user space

SYNOPSIS         top

       #include <sys/ioctl.h>

       int ioctl(int fd, int cmd, ...);

DESCRIPTION         top

       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 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 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 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;    /* Destination of copy */
               __u64 src;    /* Source 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 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_ZEROPAGE 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_ZEROPAGE 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 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 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         top

       See descriptions of the individual operations, above.

ERRORS         top

       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         top

       These ioctl(2) operations are Linux-specific.

BUGS         top

       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.

EXAMPLES         top

       See userfaultfd(2).

SEE ALSO         top

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

       Documentation/admin-guide/mm/userfaultfd.rst in the Linux kernel
       source tree

COLOPHON         top

       This page is part of release 5.11 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/.

Linux                          2021-03-22           IOCTL_USERFAULTFD(2)

Pages that refer to this page: ioctl(2)userfaultfd(2)