SWAPON(2) Linux Programmer's Manual SWAPON(2)
swapon, swapoff - start/stop swapping to file/device
#include <asm/page.h> /* to find PAGE_SIZE */
int swapon(const char *path, int swapflags);
int swapoff(const char *path);
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 argument, the new swap
area will have a higher priority than default. The priority is encoded within swapflags
(prio << SWAP_FLAG_PRIO_SHIFT) & SWAP_FLAG_PRIO_MASK
If the SWAP_FLAG_DISCARD flag is specified in the swapon() 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.
These functions may be used only by a privileged process (one having the CAP_SYS_ADMIN
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.
All priorities set with swapflags are high-priority, higher than default. 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-prior-
ity 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.
On success, zero is returned. On error, -1 is returned, and errno is set appropriately.
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;
or, for swapon(), the indicated path does not contain a valid swap signature or
resides on an in-memory filesystem like tmpfs; or, for swapoff(), path is not cur-
rently a swap area.
ENFILE The system 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. Alternatively, the maximum
number of swap files are already in use; see NOTES below.
These functions are Linux-specific and should not be used in programs intended to be por-
table. The second swapflags argument was introduced in Linux 1.3.2.
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 ker-
nel constant MAX_SWAPFILES. Before kernel 2.4.10, 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 CONFIG_MIGRATION option (which reserves two
swap table entries for the page migration features of mbind(2) and migrate_pages(2)).
Since kernel 2.6.32, the limit is further decreased by 1 if the kernel is built with the
Discard of swap pages was introduced in kernel 2.6.29, then made conditional on the
SWAP_FLAG_DISCARD flag in kernel 2.6.36, which still discards the entire swap area when
swapon() is called, even if that flag bit is not set.
mkswap(8), swapoff(8), swapon(8)
This page is part of release 3.55 of the Linux man-pages project. A description of the
project, and information about reporting bugs, can be found at
Linux 2010-11-15 SWAPON(2)