UFFDIO_REGISTER(2const)                                UFFDIO_REGISTER(2const)

NAME
       UFFDIO_REGISTER - register a memory address range with the userfaultfd
       object

LIBRARY
       Standard C library (libc, -lc)

SYNOPSIS
       #include <linux/userfaultfd.h>  /* Definition of UFFD* constants */
       #include <sys/ioctl.h>

       int ioctl(int fd, UFFDIO_REGISTER, struct uffdio_register *argp);

       #include <linux/userfaultfd.h>

       struct uffdio_range {
           __u64  start;   /* Start of range */
           __u64  len;     /* Size of range (bytes) */
       };

       struct uffdio_register {
           struct uffdio_range  range;
           __u64                mode;    /* Desired mode of operation (input) */
           __u64                ioctls;  /* Available ioctl()s (output) */
       };

DESCRIPTION
       Register a memory address range with the userfaultfd object.  The pages
       in the range must be "compatible".  Please refer to the list of
       register modes below for the compatible memory backends for each mode.

       The argp->range field defines a memory range starting at
       argp->range.start and continuing for argp->range.len bytes that should
       be handled by the userfaultfd.

       The argp->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.  Since Linux 4.3, only
              private anonymous ranges are compatible.  Since Linux 4.11,
              hugetlbfs and shared memory ranges are also compatible.

       UFFDIO_REGISTER_MODE_WP
              Track page faults on write-protected pages.  Since Linux 5.7,
              only private anonymous ranges are compatible.

       UFFDIO_REGISTER_MODE_MINOR
              Track minor page faults.  Since Linux 5.13, only hugetlbfs
              ranges are compatible.  Since Linux 5.14, compatibility with
              shmem ranges was added.

       UFFDIO_REGISTER_MODE_RWP (since Linux 7.2)
              Track page faults on read-write-protected pages.  Every access
              (read or write) to a page present within the registered range
              generates a notification once the range has been protected with
              UFFDIO_RWPROTECT(2const).  This mode cannot be combined with
              UFFDIO_REGISTER_MODE_WP; attempting to do so fails with EINVAL.
              Anonymous, shmem, and hugetlbfs ranges are compatible.

       If the operation is successful, the kernel modifies the argp->ioctls
       bit-mask field to indicate which ioctl(2) operations are available for
       the specified range.  This returned bit mask can contain the following
       bits:

       1 << _UFFDIO_COPY
              The UFFDIO_COPY operation is supported.

       1 << _UFFDIO_WAKE
              The UFFDIO_WAKE operation is supported.

       1 << _UFFDIO_WRITEPROTECT
              The UFFDIO_WRITEPROTECT operation is supported.

       1 << _UFFDIO_ZEROPAGE
              The UFFDIO_ZEROPAGE operation is supported.

       1 << _UFFDIO_CONTINUE
              The UFFDIO_CONTINUE operation is supported.

       1 << _UFFDIO_POISON
              The UFFDIO_POISON operation is supported.

       1 << _UFFDIO_RWPROTECT (since Linux 7.2)
              The UFFDIO_RWPROTECT operation is supported.  This bit is
              reported only when the range was registered with
              UFFDIO_REGISTER_MODE_RWP (which itself requires UFFD_FEATURE_RWP
              to have been negotiated).

RETURN VALUE
       On success, 0 is returned.  On error, -1 is returned and errno is set
       to indicate the error.

ERRORS
       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.

STANDARDS
       Linux.

HISTORY
       Linux 4.3.

EXAMPLES
       See userfaultfd(2).

SEE ALSO
       ioctl(2), ioctl_userfaultfd(2), UFFDIO_UNREGISTER(2const),
       userfaultfd(2)

       linux.git/Documentation/admin-guide/mm/userfaultfd.rst

Linux man-pages 6.19              2026-06-04           UFFDIO_REGISTER(2const)