.\" Copyright (C) 2003, Michael Kerrisk <mtk.manpages@gmail.com>

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.\" 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 2017-09-15 "Linux" "Linux Programmer's Manual"

.SH NAME

remap_file_pages \- create a nonlinear file mapping

.SH SYNOPSIS

.nf

.BR "#define _GNU_SOURCE" "         /* See feature_test_macros(7) */"

.B #include <sys/mman.h>

.PP

.BI "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 appropriately.

.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 CONFORMING TO

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)

.SH COLOPHON

This page is part of release 4.15 of the Linux

.I man-pages

project.

A description of the project,

information about reporting bugs,

and the latest version of this page,

can be found at

\%https://www.kernel.org/doc/man\-pages/.