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.\" Copyright (C) 2003, Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.\" 2003-12-10 Initial creation, Michael Kerrisk <mtk.manpages@gmail.com>
.\" 2004-10-28 aeb, corrected prototype, prot must be 0
.\"
.TH REMAP_FILE_PAGES 2 2021-03-22 "Linux man-pages (unreleased)"
.SH NAME
remap_file_pages \- create a nonlinear file mapping
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.BR "#define _GNU_SOURCE" " /* See feature_test_macros(7) */"
.B #include <sys/mman.h>
.PP
.BI "[[deprecated]] int remap_file_pages(void *" addr ", size_t " size \
", int " prot ,
.BI " size_t " pgoff ", int " flags );
.fi
.SH DESCRIPTION
.BR Note :
.\" commit 33041a0d76d3c3e0aff28ac95a2ffdedf1282dbc
.\" http://lwn.net/Articles/597632/
this system call was marked as deprecated starting with Linux 3.16.
In Linux 4.0, the implementation was replaced
.\" commit c8d78c1823f46519473949d33f0d1d33fe21ea16
by a slower in-kernel emulation.
Those few applications that use this system call should
consider migrating to alternatives.
This change was made because the kernel code for this system call was complex,
and it is believed to be little used or perhaps even completely unused.
While it had some use cases in database applications on 32-bit systems,
those use cases don't exist on 64-bit systems.
.PP
The
.BR remap_file_pages ()
system call is used to create a nonlinear mapping, that is, a mapping
in which the pages of the file are mapped into a nonsequential order
in memory.
The advantage of using
.BR remap_file_pages ()
over using repeated calls to
.BR mmap (2)
is that the former approach does not require the kernel to create
additional VMA (Virtual Memory Area) data structures.
.PP
To create a nonlinear mapping we perform the following steps:
.TP 3
1.
Use
.BR mmap (2)
to create a mapping (which is initially linear).
This mapping must be created with the
.B MAP_SHARED
flag.
.TP
2.
Use one or more calls to
.BR remap_file_pages ()
to rearrange the correspondence between the pages of the mapping
and the pages of the file.
It is possible to map the same page of a file
into multiple locations within the mapped region.
.PP
The
.I pgoff
and
.I size
arguments specify the region of the file that is to be relocated
within the mapping:
.I pgoff
is a file offset in units of the system page size;
.I size
is the length of the region in bytes.
.PP
The
.I addr
argument serves two purposes.
First, it identifies the mapping whose pages we want to rearrange.
Thus,
.I addr
must be an address that falls within
a region previously mapped by a call to
.BR mmap (2).
Second,
.I addr
specifies the address at which the file pages
identified by
.I pgoff
and
.I size
will be placed.
.PP
The values specified in
.I addr
and
.I size
should be multiples of the system page size.
If they are not, then the kernel rounds
.I both
values
.I down
to the nearest multiple of the page size.
.\" This rounding is weird, and not consistent with the treatment of
.\" the analogous arguments for munmap()/mprotect() and for mlock().
.\" MTK, 14 Sep 2005
.PP
The
.I prot
argument must be specified as 0.
.PP
The
.I flags
argument has the same meaning as for
.BR mmap (2),
but all flags other than
.B MAP_NONBLOCK
are ignored.
.SH RETURN VALUE
On success,
.BR remap_file_pages ()
returns 0.
On error, \-1 is returned, and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EINVAL
.I addr
does not refer to a valid mapping
created with the
.B MAP_SHARED
flag.
.TP
.B EINVAL
.IR addr ,
.IR size ,
.IR prot ,
or
.I pgoff
is invalid.
.\" And possibly others from vma->vm_ops->populate()
.SH VERSIONS
The
.BR remap_file_pages ()
system call appeared in Linux 2.5.46;
glibc support was added in version 2.3.3.
.SH STANDARDS
The
.BR remap_file_pages ()
system call is Linux-specific.
.SH NOTES
Since Linux 2.6.23,
.\" commit 3ee6dafc677a68e461a7ddafc94a580ebab80735
.BR remap_file_pages ()
creates non-linear mappings only
on in-memory filesystems such as
.BR tmpfs (5),
hugetlbfs or ramfs.
On filesystems with a backing store,
.BR remap_file_pages ()
is not much more efficient than using
.BR mmap (2)
to adjust which parts of the file are mapped to which addresses.
.SH SEE ALSO
.BR getpagesize (2),
.BR mmap (2),
.BR mmap2 (2),
.BR mprotect (2),
.BR mremap (2),
.BR msync (2)
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