.\" Copyright (c) 1992 Drew Eckhardt (drew@cs.colorado.edu), March 28, 1992

.\"

.\" %%%LICENSE_START(VERBATIM)

.\" Permission is granted to make and distribute verbatim copies of this

.\" manual provided the copyright notice and this permission notice are

.\" preserved on all copies.

.\"

.\" Permission is granted to copy and distribute modified versions of this

.\" manual under the conditions for verbatim copying, provided that the

.\" entire resulting derived work is distributed under the terms of a

.\" permission notice identical to this one.

.\"

.\" Since the Linux kernel and libraries are constantly changing, this

.\" manual page may be incorrect or out-of-date.  The author(s) assume no

.\" responsibility for errors or omissions, or for damages resulting from

.\" the use of the information contained herein.  The author(s) may not

.\" have taken the same level of care in the production of this manual,

.\" which is licensed free of charge, as they might when working

.\" professionally.

.\"

.\" Formatted or processed versions of this manual, if unaccompanied by

.\" the source, must acknowledge the copyright and authors of this work.

.\" %%%LICENSE_END

.\"

.\" Modified by Michael Haardt <michael@moria.de>

.\" Modified 1993-07-24 by Rik Faith <faith@cs.unc.edu>

.\" Modified 1995-07-22 by Michael Chastain <mec@duracef.shout.net>

.\" Modified 1995-07-23 by aeb

.\" Modified 1996-10-22 by Eric S. Raymond <esr@thyrsus.com>

.\" Modified 1998-09-08 by aeb

.\" Modified 2004-06-17 by Michael Kerrisk <mtk.manpages@gmail.com>

.\" Modified 2004-10-10 by aeb

.\" 2004-12-14 mtk, Anand Kumria: added new errors

.\" 2007-06-22 Ivana Varekova <varekova@redhat.com>, mtk

.\"     Update text describing limit on number of swap files.

.\"

.\" FIXME Linux 3.11 added SWAP_FLAG_DISCARD_ONCE and SWAP_FLAG_DISCARD_PAGES

.\"	commit dcf6b7ddd7df8965727746f89c59229b23180e5a

.\"	Author: Rafael Aquini <aquini@redhat.com>

.\"	Date:   Wed Jul 3 15:02:46 2013 -0700

.\"

.TH SWAPON 2 2017-09-15 "Linux" "Linux Programmer's Manual"

.SH NAME

swapon, swapoff \- start/stop swapping to file/device

.SH SYNOPSIS

.B #include <unistd.h>

.br

.B #include <sys/swap.h>

.PP

.BI "int swapon(const char *" path ", int " swapflags );

.br

.BI "int swapoff(const char *" path );

.SH DESCRIPTION

.BR swapon ()

sets the swap area to the file or block device specified by

.IR path .

.BR swapoff ()

stops swapping to the file or block device specified by

.IR path .

.PP

If the

.B SWAP_FLAG_PREFER

flag is specified in the

.BR swapon ()

.I swapflags

argument, the new swap area will have a higher priority than default.

The priority is encoded within

.I swapflags

as:

.PP

.in +4n

.EX

.I "(prio << SWAP_FLAG_PRIO_SHIFT) & SWAP_FLAG_PRIO_MASK"

.EE

.in

.PP

If the

.B SWAP_FLAG_DISCARD

flag is specified in the

.BR swapon ()

.I swapflags

argument, freed swap pages will be discarded before they are reused,

if the swap device supports the discard or trim operation.

(This may improve performance on some Solid State Devices,

but often it does not.)

See also NOTES.

.PP

These functions may be used only by a privileged process (one having the

.B CAP_SYS_ADMIN

capability).

.SS Priority

Each swap area has a priority, either high or low.

The default priority is low.

Within the low-priority areas,

newer areas are even lower priority than older areas.

.PP

All priorities set with

.I swapflags

are high-priority, higher than default.

They may have any nonnegative value chosen by the caller.

Higher numbers mean higher priority.

.PP

Swap pages are allocated from areas in priority order,

highest priority first.

For areas with different priorities,

a higher-priority area is exhausted before using a lower-priority area.

If two or more areas have the same priority,

and it is the highest priority available,

pages are allocated on a round-robin basis between them.

.PP

As of Linux 1.3.6, the kernel usually follows these rules,

but there are exceptions.

.SH RETURN VALUE

On success, zero is returned.

On error, \-1 is returned, and

.I errno

is set appropriately.

.SH ERRORS

.TP

.B EBUSY

(for

.BR swapon ())

The specified

.I path

is already being used as a swap area.

.TP

.B EINVAL

The file

.I path

exists, but refers neither to a regular file nor to a block device;

.TP

.B EINVAL

.RB ( swapon ())

The indicated path does not contain a valid swap signature or

resides on an in-memory filesystem such as

.BR tmpfs (5).

.TP

.BR EINVAL " (since Linux 3.4)"

.RB ( swapon ())

An invalid flag value was specified in

.IR flags .

.TP

.B EINVAL

.RB ( swapoff ())

.I path

is not currently a swap area.

.TP

.B ENFILE

The system-wide limit on the total number of open files has been reached.

.TP

.B ENOENT

The file

.I path

does not exist.

.TP

.B ENOMEM

The system has insufficient memory to start swapping.

.TP

.B EPERM

The caller does not have the

.B CAP_SYS_ADMIN

capability.

Alternatively, the maximum number of swap files are already in use;

see NOTES below.

.SH CONFORMING TO

These functions are Linux-specific and should not be used in programs

intended to be portable.

The second

.I swapflags

argument was introduced in Linux 1.3.2.

.SH NOTES

The partition or path must be prepared with

.BR mkswap (8).

.PP

There is an upper limit on the number of swap files that may be used,

defined by the kernel constant

.BR MAX_SWAPFILES .

Before kernel 2.4.10,

.B MAX_SWAPFILES

has the value 8;

since kernel 2.4.10, it has the value 32.

Since kernel 2.6.18, the limit is decreased by 2 (thus: 30)

if the kernel is built with the

.B CONFIG_MIGRATION

option

(which reserves two swap table entries for the page migration features of

.BR mbind (2)

and

.BR migrate_pages (2)).

Since kernel 2.6.32, the limit is further decreased by 1

if the kernel is built with the

.B CONFIG_MEMORY_FAILURE

option.

.PP

Discard of swap pages was introduced in kernel 2.6.29,

then made conditional

on the

.B SWAP_FLAG_DISCARD

flag in kernel 2.6.36,

.\" To be precise: 2.6.35.5

which still discards the

entire swap area when

.BR swapon ()

is called, even if that flag bit is not set.

.SH SEE ALSO

.BR mkswap (8),

.BR swapoff (8),

.BR swapon (8)

.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/.