.\" Copyright, the authors of the Linux man-pages project
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.TH UFFDIO_SET_MODE 2const 2026-05-28 "Linux man-pages 6.19"
.SH NAME
UFFDIO_SET_MODE
\-
toggle userfaultfd runtime mode bits
.SH LIBRARY
Standard C library
.RI ( libc ,\~ \-lc )
.SH SYNOPSIS
.nf
.BR "#include <linux/userfaultfd.h>" "  /* Definition of " UFFD* " constants */"
.B #include <sys/ioctl.h>
.P
.BI "int ioctl(int " fd ", UFFDIO_SET_MODE, struct uffdio_set_mode *" argp );
.P
.B #include <linux/userfaultfd.h>
.P
.fi
.EX
.B struct uffdio_set_mode {
.BR "    __u64  enable;" "   /* Feature bits to set */"
.BR "    __u64  disable;" "  /* Feature bits to clear */"
.B };
.EE
.SH DESCRIPTION
Toggle userfaultfd features that may be flipped at runtime.
.P
Bits set in
.I .enable
turn the named features on;
bits set in
.I .disable
turn them off.
The two fields must not overlap.
Today only
.B UFFD_FEATURE_RWP_ASYNC
is a valid bit in either field;
any other bit causes the ioctl to fail with
.BR EINVAL .
Enabling
.B UFFD_FEATURE_RWP_ASYNC
also requires
.B UFFD_FEATURE_RWP
to have been negotiated at
.BR UFFDIO_API (2const)
time.
.P
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.
.SH RETURN VALUE
On success,
0 is returned.
On error, \-1 is returned and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EINVAL
A bit other than
.B UFFD_FEATURE_RWP_ASYNC
was specified in
.I .enable
or
.IR .disable .
.TP
.B EINVAL
The fields
.I .enable
and
.I .disable
overlap.
.TP
.B EINVAL
.B UFFD_FEATURE_RWP_ASYNC
was requested without
.B UFFD_FEATURE_RWP
having been negotiated.
.TP
.B EFAULT
.I argp
refers to an address that is outside the calling process's
accessible address space.
.SH STANDARDS
Linux.
.SH HISTORY
Linux 7.2.
.SH EXAMPLES
See
.BR userfaultfd (2).
.SH SEE ALSO
.BR ioctl (2),
.BR ioctl_userfaultfd (2),
.BR userfaultfd (2)
.P
.I linux.git/\:Documentation/\:admin\-guide/\:mm/\:userfaultfd.rst