mirror of https://github.com/mkerrisk/man-pages
userfaultfd.2: New page describing userfaultfd(2) system call.
Reviewed-by: Andrea Arcangeli <aarcange@redhat.com> Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com> Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
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Mike Rapoport
Michael Kerrisk
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.\" Copyright (c) 2016, IBM Corporation.
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.\" Written by Mike Rapoport <rppt@linux.vnet.ibm.com>
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.\"
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.\" %%%LICENSE_START(VERBATIM)
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.\" Permission is granted to make and distribute verbatim copies of this
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.\" manual provided the copyright notice and this permission notice are
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.\" preserved on all copies.
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.\"
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.\" Permission is granted to copy and distribute modified versions of this
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.\" manual under the conditions for verbatim copying, provided that the
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.\" entire resulting derived work is distributed under the terms of a
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.\" permission notice identical to this one.
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.\"
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.\" Since the Linux kernel and libraries are constantly changing, this
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.\" manual page may be incorrect or out-of-date. The author(s) assume no
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.\" responsibility for errors or omissions, or for damages resulting from
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.\" the use of the information contained herein. The author(s) may not
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.\" have taken the same level of care in the production of this manual,
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.\" which is licensed free of charge, as they might when working
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.\" professionally.
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.\"
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.\" Formatted or processed versions of this manual, if unaccompanied by
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.\" the source, must acknowledge the copyright and authors of this work.
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.\" %%%LICENSE_END
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.\"
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.TH USERFAULTFD 2 2016-12-12 "Linux" "Linux Programmer's Manual"
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.SH NAME
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userfaultfd \- create a file descriptor for handling page faults in user
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space
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.SH SYNOPSIS
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.nf
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.B #include <sys/types.h>
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.sp
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.BI "int userfaultfd(int " flags );
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.fi
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.PP
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.IR Note :
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There is no glibc wrapper for this system call; see NOTES.
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.SH DESCRIPTION
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.BR userfaultfd (2)
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creates a userfaultfd object that can be used for delegation of page fault
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handling to a user space application.
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The userfaultfd should be configured using
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.BR ioctl (2).
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Once the userfaultfd is configured, the application can use
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.BR read (2)
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to receive userfaultfd notifications.
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The reads from userfaultfd may be blocking or non-blocking, depending on
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the value of
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.I flags
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used for the creation of the userfaultfd or subsequent calls to
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.BR fcntl (2) .
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The following values may be bitwise ORed in
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.IR flags
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to change the behavior of
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.BR userfaultfd ():
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.TP
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.BR O_CLOEXEC
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Enable the close-on-exec flag for the new userfaultfd object.
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See the description of the
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.B O_CLOEXEC
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flag in
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.BR open (2)
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.TP
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.BR O_NONBLOCK
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Enables non-blocking operation for the userfaultfd
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.BR O_NONBLOCK
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See the description of the
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.BR O_NONBLOCK
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flag in
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.BR open (2).
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.\"
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.SS Userfaultfd operation
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After the userfaultfd object is created with
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.BR userfaultfd (2)
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system call, the application have to enable it using
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.I UFFDIO_API
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ioctl to perform API version and supported features handshake between the
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kernel and the user space.
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If the
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.I UFFDIO_API
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is successful, the application should register memory ranges using
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.I UFFDIO_REGISTER
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ioctl. After successful completion of
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.I UFFDIO_REGISTER
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ioctl, a page fault occurring in the requested memory range, and satisfying
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the mode defined at the register time, will be forwarded by the kernel to
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the user space application.
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The application then can use
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.I UFFDIO_COPY
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or
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.I UFFDIO_ZERO
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ioctls to resolve the page fault.
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.PP
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Currently, userfaultfd can only be used with anonymous private memory
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mappings.
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.\"
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.SS API Ioctls
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The API ioctls are used to configure userfaultfd behavior.
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They allow to choose what features will be enabled and what kinds of events
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will be delivered to the application.
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.TP
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.BR "UFFDIO_API struct uffdio_api *" api
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Enable userfaultfd and perform API handshake.
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The
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.I uffdio_api
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structure is defined as:
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.in +4n
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.nf
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struct uffdio_api {
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__u64 api;
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__u64 features;
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__u64 ioctls;
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};
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.fi
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.in
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The
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.I api
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field denotes the API version requested by the application.
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The kernel verifies that it can support the required API, and sets the
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.I features
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and
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.I ioctls
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fields to bit masks representing all the available features and the generic
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ioctls available.
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.\"
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.TP
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.BI "UFFDIO_REGISTER struct uffdio_register *" arg
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Register a memory range with userfaultfd.
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The
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.I uffdio_register
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structure is defined as:
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.in +4n
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.nf
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struct uffdio_range {
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__u64 start;
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__u64 end;
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};
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struct uffdio_register {
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struct uffdio_range range;
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__u64 mode;
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__u64 ioctls;
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};
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.fi
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.in
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The
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.I range
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field defines a memory range starting at
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.I start
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and ending at
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.I end
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that should be handled by the userfaultfd.
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The
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.I mode
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defines mode of operation desired for this memory region.
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The following values may be bitwise ORed to set the userfaultfd mode for
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particular range:
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.RS
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.sp
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.PD 0
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.TP 12
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.B UFFDIO_REGISTER_MODE_MISSING
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Track page faults on missing pages
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.TP 12
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.B UFFDIO_REGISTER_MODE_WP
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Track page faults on write protected pages.
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Currently the only supported mode is
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.I UFFDIO_REGISTER_MODE_MISSING
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.PD
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.RE
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.IP
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The kernel answers which ioctl commands are available for the requested
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range in the
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.I ioctls
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field.
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.\"
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.TP
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.BI "UFFDIO_UNREGISTER struct uffdio_register *" arg
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Unregister a memory range from userfaultfd.
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.\"
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.SS Range Ioctls
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The range ioctls enable the calling application to resolve page fault
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events in consistent way.
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.TP
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.BI "UFFDIO_COPY struct uffdio_copy *" arg
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Atomically copy a continuous memory chunk into the userfault registered
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range and optionally wake up the blocked thread.
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The source and destination addresses and the amount of bytes to copy are
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specified by
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.IR src ", " dst ", and " len
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fields of
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.I "struct uffdio_copy"
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respectively:
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.in +4n
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.nf
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struct uffdio_copy {
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__u64 dst;
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__u64 src;
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__u64 len;
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__u64 mode;
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__s64 copy;
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};
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.nf
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.fi
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The following values may be bitwise ORed in
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.IR mode
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to change the behavior of
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.I UFFDIO_COPY
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ioctl:
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.RS
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.sp
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.PD 0
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.TP 12
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.B UFFDIO_COPY_MODE_DONTWAKE
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Do not wake up the thread that waits for page fault resolution
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.PD
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.RE
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.IP
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The
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.I copy
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field of the
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.I uffdio_copy
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structure is used by the kernel to return amount of bytes that was actually
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copied, or an error.
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If
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.I uffdio_copy.copy
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doesn't match the
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.I uffdio_copy.len
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passed in input to
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.IR UFFDIO_COPY ,
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the ioctl will return
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.BR -EAGAIN .
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If the ioctl returns zero it means it succeeded, no error was reported and
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the entire area was copied.
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If a an invalid fault happens while writing to the
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.I uffdio_copy.copy
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field, the syscall will return
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.BR -EFAULT .
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.I uffdio_copy.copy
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is an output-only field so it is not being read by the UFFDIO_COPY ioctl.
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.\"
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.TP
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.BI "UFFDIO_ZERO struct uffdio_zero *" arg
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Zero out a part of memory range registered with userfaultfd.
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The requested range is specified by
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.I range
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field of
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.I uffdio_zeropage
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structure:
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.in +4n
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.nf
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struct uffdio_zeropage {
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struct uffdio_range range;
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__u64 mode;
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__s64 zeropage;
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};
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.nf
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.fi
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The following values may be bitwise ORed in
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.IR mode
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to change the behavior of
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.I UFFDIO_ZERO
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ioctl:
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.RS
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.sp
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.PD 0
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.TP 12
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.B UFFDIO_ZEROPAGE_MODE_DONTWAKE
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Do not wake up the thread that waits for page fault resolution
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.PD
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.RE
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.IP
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The
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.I zeropage
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field of the
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.I uffdio_zero
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structure is used by the kernel to return amount of bytes that was actually
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zeroed, or an error the same way like
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.IR uffdio_copy.copy .
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.\"
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.TP
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.BI "UFFDIO_WAKE struct uffdio_range *" arg
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Wake up the thread waiting for the page fault resolution.
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.SH RETURN VALUE
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For a successful call, the
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.BR userfaultfd (2)
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system call returns the new file descriptor for the userfaultfd object.
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On error, \-1 is returned, and
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.I errno
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is set appropriately.
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.SH ERRORS
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.TP
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.B EINVAL
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An unsupported value was specified in
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.IR flags .
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.TP
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.BR EMFILE
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The per-process limit on the number of open file descriptors has been
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reached
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.TP
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.B ENFILE
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The system-wide limit on the total number of open files has been
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reached.
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.TP
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.B ENOMEM
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Insufficient kernel memory was available.
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.SH CONFORMING TO
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.BR userfaultfd ()
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is Linux-specific and should not be used in programs intended to be
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portable.
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.SH NOTES
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Glibc does not provide a wrapper for this system call; call it using
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.BR syscall (2).
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.SH SEE ALSO
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.BR fcntl (2),
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.BR ioctl (2)
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.IR Documentation/vm/userfaultfd.txt
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in the Linux kernel source tree
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