swapon(2) System Calls Manual swapon(2)
NAME
swapon, swapoff - start/stop swapping to file/device
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <sys/swap.h>
int swapon(const char *path, int swapflags);
int swapoff(const char *path);
DESCRIPTION
swapon() sets the swap area to the file or block device specified by
path. swapoff() stops swapping to the file or block device specified by
path.
If the SWAP_FLAG_PREFER flag is specified in the swapon() swapflags ar-
gument, the new swap area will have a higher priority than default. The
priority is encoded within swapflags as:
(prio << SWAP_FLAG_PRIO_SHIFT) & SWAP_FLAG_PRIO_MASK
If the SWAP_FLAG_DISCARD flag is specified in the swapon() swapflags ar-
gument, freed swap pages will be discarded before they are reused, if
the swap device supports the discard or trim operation. (This may im-
prove performance on some Solid State Devices, but often it does not.)
See also NOTES.
These functions may be used only by a privileged process (one having the
CAP_SYS_ADMIN capability).
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 pri-
ority than older areas.
All priorities set with swapflags are high-priority, higher than de-
fault. They may have any nonnegative value chosen by the caller.
Higher numbers mean higher priority.
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.
As of Linux 1.3.6, the kernel usually follows these rules, but there are
exceptions.
RETURN VALUE
On success, zero is returned. On error, -1 is returned, and errno is
set to indicate the error.
ERRORS
EBUSY (for swapon()) The specified path is already being used as a swap
area.
EINVAL The file path exists, but refers neither to a regular file nor to
a block device;
EINVAL (swapon()) The indicated path does not contain a valid swap sig-
nature or resides on an in-memory filesystem such as tmpfs(5).
EINVAL (since Linux 3.4)
(swapon()) An invalid flag value was specified in swapflags.
EINVAL (swapoff()) path is not currently a swap area.
ENFILE The system-wide limit on the total number of open files has been
reached.
ENOENT The file path does not exist.
ENOMEM The system has insufficient memory to start swapping.
EPERM The caller does not have the CAP_SYS_ADMIN capability. Alterna-
tively, the maximum number of swap files are already in use; see
NOTES below.
STANDARDS
Linux.
HISTORY
The swapflags argument was introduced in Linux 1.3.2.
NOTES
The partition or path must be prepared with mkswap(8).
There is an upper limit on the number of swap files that may be used,
defined by the kernel constant MAX_SWAPFILES. Before Linux 2.4.10,
MAX_SWAPFILES has the value 8; since Linux 2.4.10, it has the value 32.
Since Linux 2.6.18, the limit is decreased by 2 (thus 30), since Linux
5.19, the limit is decreased by 3 (thus: 29) if the kernel is built with
the CONFIG_MIGRATION option (which reserves two swap table entries for
the page migration features of mbind(2) and migrate_pages(2)). Since
Linux 2.6.32, the limit is further decreased by 1 if the kernel is built
with the CONFIG_MEMORY_FAILURE option. Since Linux 5.14, the limit is
further decreased by 4 if the kernel is built with the CONFIG_DE-
VICE_PRIVATE option. Since Linux 5.19, the limit is further decreased
by 1 if the kernel is built with the CONFIG_PTE_MARKER option.
Discard of swap pages was introduced in Linux 2.6.29, then made condi-
tional on the SWAP_FLAG_DISCARD flag in Linux 2.6.36, which still dis-
cards the entire swap area when swapon() is called, even if that flag
bit is not set.
SEE ALSO
mkswap(8), swapoff(8), swapon(8)
Linux man-pages 6.9.1 2024-05-02 swapon(2)
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