man2/swapon.2

   1 .\" Copyright (c) 1992 Drew Eckhardt (drew@cs.colorado.edu), March 28, 1992
   2 .\"
   3 .\" SPDX-License-Identifier: Linux-man-pages-copyleft
   4 .\"
   5 .\" Modified by Michael Haardt <michael@moria.de>
   6 .\" Modified 1993-07-24 by Rik Faith <faith@cs.unc.edu>
   7 .\" Modified 1995-07-22 by Michael Chastain <mec@duracef.shout.net>
   8 .\" Modified 1995-07-23 by aeb
   9 .\" Modified 1996-10-22 by Eric S. Raymond <esr@thyrsus.com>
  10 .\" Modified 1998-09-08 by aeb
  11 .\" Modified 2004-06-17 by Michael Kerrisk <mtk.manpages@gmail.com>
  12 .\" Modified 2004-10-10 by aeb
  13 .\" 2004-12-14 mtk, Anand Kumria: added new errors
  14 .\" 2007-06-22 Ivana Varekova <varekova@redhat.com>, mtk
  15 .\"     Update text describing limit on number of swap files.
  16 .\" 2021-01-17 Alex Baranowski <alex@euro-linux.com>
  17 .\"     Update information about available swap files decreased by
  18 .\"     CONFIG_DEVICE_PRIVATE option.
  19 .\"
  20 .\" FIXME Linux 3.11 added SWAP_FLAG_DISCARD_ONCE and SWAP_FLAG_DISCARD_PAGES
  21 .\"     commit dcf6b7ddd7df8965727746f89c59229b23180e5a
  22 .\"     Author: Rafael Aquini <aquini@redhat.com>
  23 .\"     Date:   Wed Jul 3 15:02:46 2013 -0700
  24 .\"
  25 .TH SWAPON 2 2021-03-22 "Linux man-pages (unreleased)" "Linux Programmer's Manual"
  26 .SH NAME
  27 swapon, swapoff \- start/stop swapping to file/device
  28 .SH LIBRARY
  29 Standard C library
  30 .RI ( libc ", " \-lc )
  31 .SH SYNOPSIS
  32 .nf
  33 .B #include <sys/swap.h>
  34 .PP
  35 .BI "int swapon(const char *" path ", int " swapflags );
  36 .BI "int swapoff(const char *" path );
  37 .fi
  38 .SH DESCRIPTION
  39 .BR swapon ()
  40 sets the swap area to the file or block device specified by
  41 .IR path .
  42 .BR swapoff ()
  43 stops swapping to the file or block device specified by
  44 .IR path .
  45 .PP
  46 If the
  47 .B SWAP_FLAG_PREFER
  48 flag is specified in the
  49 .BR swapon ()
  50 .I swapflags
  51 argument, the new swap area will have a higher priority than default.
  52 The priority is encoded within
  53 .I swapflags
  54 as:
  55 .PP
  56 .in +4n
  57 .EX
  58 .I "(prio << SWAP_FLAG_PRIO_SHIFT) & SWAP_FLAG_PRIO_MASK"
  59 .EE
  60 .in
  61 .PP
  62 If the
  63 .B SWAP_FLAG_DISCARD
  64 flag is specified in the
  65 .BR swapon ()
  66 .I swapflags
  67 argument, freed swap pages will be discarded before they are reused,
  68 if the swap device supports the discard or trim operation.
  69 (This may improve performance on some Solid State Devices,
  70 but often it does not.)
  71 See also NOTES.
  72 .PP
  73 These functions may be used only by a privileged process (one having the
  74 .B CAP_SYS_ADMIN
  75 capability).
  76 .SS Priority
  77 Each swap area has a priority, either high or low.
  78 The default priority is low.
  79 Within the low-priority areas,
  80 newer areas are even lower priority than older areas.
  81 .PP
  82 All priorities set with
  83 .I swapflags
  84 are high-priority, higher than default.
  85 They may have any nonnegative value chosen by the caller.
  86 Higher numbers mean higher priority.
  87 .PP
  88 Swap pages are allocated from areas in priority order,
  89 highest priority first.
  90 For areas with different priorities,
  91 a higher-priority area is exhausted before using a lower-priority area.
  92 If two or more areas have the same priority,
  93 and it is the highest priority available,
  94 pages are allocated on a round-robin basis between them.
  95 .PP
  96 As of Linux 1.3.6, the kernel usually follows these rules,
  97 but there are exceptions.
  98 .SH RETURN VALUE
  99 On success, zero is returned.
 100 On error, \-1 is returned, and
 101 .I errno
 102 is set to indicate the error.
 103 .SH ERRORS
 104 .TP
 105 .B EBUSY
 106 (for
 107 .BR swapon ())
 108 The specified
 109 .I path
 110 is already being used as a swap area.
 111 .TP
 112 .B EINVAL
 113 The file
 114 .I path
 115 exists, but refers neither to a regular file nor to a block device;
 116 .TP
 117 .B EINVAL
 118 .RB ( swapon ())
 119 The indicated path does not contain a valid swap signature or
 120 resides on an in-memory filesystem such as
 121 .BR tmpfs (5).
 122 .TP
 123 .BR EINVAL " (since Linux 3.4)"
 124 .RB ( swapon ())
 125 An invalid flag value was specified in
 126 .IR swapflags .
 127 .TP
 128 .B EINVAL
 129 .RB ( swapoff ())
 130 .I path
 131 is not currently a swap area.
 132 .TP
 133 .B ENFILE
 134 The system-wide limit on the total number of open files has been reached.
 135 .TP
 136 .B ENOENT
 137 The file
 138 .I path
 139 does not exist.
 140 .TP
 141 .B ENOMEM
 142 The system has insufficient memory to start swapping.
 143 .TP
 144 .B EPERM
 145 The caller does not have the
 146 .B CAP_SYS_ADMIN
 147 capability.
 148 Alternatively, the maximum number of swap files are already in use;
 149 see NOTES below.
 150 .SH STANDARDS
 151 These functions are Linux-specific and should not be used in programs
 152 intended to be portable.
 153 The second
 154 .I swapflags
 155 argument was introduced in Linux 1.3.2.
 156 .SH NOTES
 157 The partition or path must be prepared with
 158 .BR mkswap (8).
 159 .PP
 160 There is an upper limit on the number of swap files that may be used,
 161 defined by the kernel constant
 162 .BR MAX_SWAPFILES .
 163 Before kernel 2.4.10,
 164 .B MAX_SWAPFILES
 165 has the value 8;
 166 since kernel 2.4.10, it has the value 32.
 167 Since kernel 2.6.18, the limit is decreased by 2 (thus: 30)
 168 if the kernel is built with the
 169 .B CONFIG_MIGRATION
 170 option
 171 (which reserves two swap table entries for the page migration features of
 172 .BR mbind (2)
 173 and
 174 .BR migrate_pages (2)).
 175 Since kernel 2.6.32, the limit is further decreased by 1
 176 if the kernel is built with the
 177 .B CONFIG_MEMORY_FAILURE
 178 option.
 179 Since kernel 5.14, the limit is further decreased by 4
 180 if the kernel is built with the
 181 .B CONFIG_DEVICE_PRIVATE
 182 option.
 183 .PP
 184 Discard of swap pages was introduced in kernel 2.6.29,
 185 then made conditional
 186 on the
 187 .B SWAP_FLAG_DISCARD
 188 flag in kernel 2.6.36,
 189 .\" To be precise: 2.6.35.5
 190 which still discards the
 191 entire swap area when
 192 .BR swapon ()
 193 is called, even if that flag bit is not set.
 194 .SH SEE ALSO
 195 .BR mkswap (8),
 196 .BR swapoff (8),
 197 .BR swapon (8)