UFFDIO_SET_MODE(2const)                                UFFDIO_SET_MODE(2const)

NAME
       UFFDIO_SET_MODE - toggle userfaultfd runtime mode bits

LIBRARY
       Standard C library (libc, -lc)

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

       int ioctl(int fd, UFFDIO_SET_MODE, struct uffdio_set_mode *argp);

       #include <linux/userfaultfd.h>

       struct uffdio_set_mode {
           __u64  enable;   /* Feature bits to set */
           __u64  disable;  /* Feature bits to clear */
       };

DESCRIPTION
       Toggle userfaultfd features that may be flipped at runtime.

       Bits set in .enable turn the named features on; bits set in .disable
       turn them off.  The two fields must not overlap.  Today only
       UFFD_FEATURE_RWP_ASYNC is a valid bit in either field; any other bit
       causes the ioctl to fail with EINVAL.  Enabling UFFD_FEATURE_RWP_ASYNC
       also requires UFFD_FEATURE_RWP to have been negotiated at
       UFFDIO_API(2const) time.

       The operation is serialized against in-flight page faults, so the new
       mode takes effect only after every fault that started before the call
       has finished, and any fault that starts after the call observes the new
       mode.  This allows a single userfaultfd to switch between lightweight
       async detection and synchronous eviction without re-registering its
       ranges.

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

ERRORS
       EINVAL A bit other than UFFD_FEATURE_RWP_ASYNC was specified in .enable
              or .disable.

       EINVAL The fields .enable and .disable overlap.

       EINVAL UFFD_FEATURE_RWP_ASYNC was requested without UFFD_FEATURE_RWP
              having been negotiated.

       EFAULT argp refers to an address that is outside the calling process's
              accessible address space.

STANDARDS
       Linux.

HISTORY
       Linux 7.2.

EXAMPLES
       See userfaultfd(2).

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

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

Linux man-pages 6.19              2026-05-28           UFFDIO_SET_MODE(2const)