-.\" Hey Emacs! This file is -*- nroff -*- source.
-.\"
.\" 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.
.\"
.\" 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 1995-07-23 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@gmx.net>
+.\" 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.
.\"
-.TH SWAPON 2 2007-06-22 "Linux" "Linux Programmer's Manual"
+.\" 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 <asm/page.h> /* to find PAGE_SIZE */
-.br
.B #include <sys/swap.h>
-.sp
+.PP
.BI "int swapon(const char *" path ", int " swapflags );
.br
.BI "int swapoff(const char *" path );
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 ()
-takes a
-.I swapflags
-argument.
-If
.I swapflags
-has the
-.B SWAP_FLAG_PREFER
-bit turned on, the new swap area will have a higher priority than default.
+argument, the new swap area will have a higher priority than default.
The priority is encoded within
.I swapflags
as:
-.br
-.sp
-.I " (prio << SWAP_FLAG_PRIO_SHIFT) & SWAP_FLAG_PRIO_MASK"
-.br
.PP
-These functions may only be used by a privileged process (one having the
+.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
All priorities set with
.I swapflags
are high-priority, higher than default.
-They may have any non-negative value chosen by the caller.
+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,
.PP
As of Linux 1.3.6, the kernel usually follows these rules,
but there are exceptions.
-.SH "RETURN VALUE"
+.SH RETURN VALUE
On success, zero is returned.
On error, \-1 is returned, and
.I errno
The file
.I path
exists, but refers neither to a regular file nor to a block device;
-or, for
-.BR swapon (),
-the indicated path does not contain a valid swap signature;
-or, for
-.BR swapoff (),
+.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 limit on the total number of open files has been reached.
+The system-wide limit on the total number of open files has been reached.
.TP
.B ENOENT
The file
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
+.SH CONFORMING TO
+These functions are Linux-specific and should not be used in programs
intended to be portable.
The second
.I swapflags
.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 MAX_SWAPFILES.
-Before kernel 2.6.10, MAX_SWAPFILES has the value 8;
-since kernel 2.6.10, it has the value 32.
+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
.BR mbind (2)
and
.BR migrate_pages (2)).
-.SH "SEE ALSO"
+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)
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