swapon(2) System Calls Manual swapon(2)

swapon, swapoff - start/stop swapping to file/device

Standard C library (libc, -lc)

#include <sys/swap.h>
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 as:


(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 capability).

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-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.

On success, zero is returned. On error, -1 is returned, and errno is set to indicate the error.

(for swapon()) The specified path is already being used as a swap area.
The file path exists, but refers neither to a regular file nor to a block device;
(swapon()) The indicated path does not contain a valid swap signature or resides on an in-memory filesystem such as tmpfs(5).
(swapon()) An invalid flag value was specified in swapflags.
(swapoff()) path is not currently a swap area.
The system-wide limit on the total number of open files has been reached.
The file path does not exist.
The system has insufficient memory to start swapping.
The caller does not have the CAP_SYS_ADMIN capability. Alternatively, the maximum number of swap files are already in use; see NOTES below.

Linux.

The 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 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_DEVICE_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 conditional on the SWAP_FLAG_DISCARD flag in Linux 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)

2023-12-22 Linux man-pages 6.7