1 .\" Copyright (c) 1992 Drew Eckhardt (drew@cs.colorado.edu), March 28, 1992
3 .\" %%%LICENSE_START(VERBATIM)
4 .\" Permission is granted to make and distribute verbatim copies of this
5 .\" manual provided the copyright notice and this permission notice are
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8 .\" Permission is granted to copy and distribute modified versions of this
9 .\" manual under the conditions for verbatim copying, provided that the
10 .\" entire resulting derived work is distributed under the terms of a
11 .\" permission notice identical to this one.
13 .\" Since the Linux kernel and libraries are constantly changing, this
14 .\" manual page may be incorrect or out-of-date. The author(s) assume no
15 .\" responsibility for errors or omissions, or for damages resulting from
16 .\" the use of the information contained herein. The author(s) may not
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18 .\" which is licensed free of charge, as they might when working
21 .\" Formatted or processed versions of this manual, if unaccompanied by
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25 .\" Modified by Michael Haardt <michael@moria.de>
26 .\" Modified 1993-07-24 by Rik Faith <faith@cs.unc.edu>
27 .\" Modified 1995-07-22 by Michael Chastain <mec@duracef.shout.net>
28 .\" Modified 1995-07-23 by aeb
29 .\" Modified 1996-10-22 by Eric S. Raymond <esr@thyrsus.com>
30 .\" Modified 1998-09-08 by aeb
31 .\" Modified 2004-06-17 by Michael Kerrisk <mtk.manpages@gmail.com>
32 .\" Modified 2004-10-10 by aeb
33 .\" 2004-12-14 mtk, Anand Kumria: added new errors
34 .\" 2007-06-22 Ivana Varekova <varekova@redhat.com>, mtk
35 .\" Update text describing limit on number of swap files.
37 .\" FIXME Linux 3.11 added SWAP_FLAG_DISCARD_ONCE and SWAP_FLAG_DISCARD_PAGES
38 .\" commit dcf6b7ddd7df8965727746f89c59229b23180e5a
39 .\" Author: Rafael Aquini <aquini@redhat.com>
40 .\" Date: Wed Jul 3 15:02:46 2013 -0700
42 .TH SWAPON 2 2021-03-22 "Linux" "Linux Programmer's Manual"
44 swapon, swapoff \- start/stop swapping to file/device
47 .B #include <sys/swap.h>
49 .BI "int swapon(const char *" path ", int " swapflags );
50 .BI "int swapoff(const char *" path );
54 sets the swap area to the file or block device specified by
57 stops swapping to the file or block device specified by
62 flag is specified in the
65 argument, the new swap area will have a higher priority than default.
66 The priority is encoded within
72 .I "(prio << SWAP_FLAG_PRIO_SHIFT) & SWAP_FLAG_PRIO_MASK"
78 flag is specified in the
81 argument, freed swap pages will be discarded before they are reused,
82 if the swap device supports the discard or trim operation.
83 (This may improve performance on some Solid State Devices,
84 but often it does not.)
87 These functions may be used only by a privileged process (one having the
91 Each swap area has a priority, either high or low.
92 The default priority is low.
93 Within the low-priority areas,
94 newer areas are even lower priority than older areas.
96 All priorities set with
98 are high-priority, higher than default.
99 They may have any nonnegative value chosen by the caller.
100 Higher numbers mean higher priority.
102 Swap pages are allocated from areas in priority order,
103 highest priority first.
104 For areas with different priorities,
105 a higher-priority area is exhausted before using a lower-priority area.
106 If two or more areas have the same priority,
107 and it is the highest priority available,
108 pages are allocated on a round-robin basis between them.
110 As of Linux 1.3.6, the kernel usually follows these rules,
111 but there are exceptions.
113 On success, zero is returned.
114 On error, \-1 is returned, and
116 is set to indicate the error.
124 is already being used as a swap area.
129 exists, but refers neither to a regular file nor to a block device;
133 The indicated path does not contain a valid swap signature or
134 resides on an in-memory filesystem such as
137 .BR EINVAL " (since Linux 3.4)"
139 An invalid flag value was specified in
145 is not currently a swap area.
148 The system-wide limit on the total number of open files has been reached.
156 The system has insufficient memory to start swapping.
159 The caller does not have the
162 Alternatively, the maximum number of swap files are already in use;
165 These functions are Linux-specific and should not be used in programs
166 intended to be portable.
169 argument was introduced in Linux 1.3.2.
171 The partition or path must be prepared with
174 There is an upper limit on the number of swap files that may be used,
175 defined by the kernel constant
177 Before kernel 2.4.10,
180 since kernel 2.4.10, it has the value 32.
181 Since kernel 2.6.18, the limit is decreased by 2 (thus: 30)
182 if the kernel is built with the
185 (which reserves two swap table entries for the page migration features of
188 .BR migrate_pages (2)).
189 Since kernel 2.6.32, the limit is further decreased by 1
190 if the kernel is built with the
191 .B CONFIG_MEMORY_FAILURE
194 Discard of swap pages was introduced in kernel 2.6.29,
195 then made conditional
198 flag in kernel 2.6.36,
199 .\" To be precise: 2.6.35.5
200 which still discards the
201 entire swap area when
203 is called, even if that flag bit is not set.