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