swapon(2) — Linux manual page

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 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).

   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 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.

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 signature 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.
              Alternatively, 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_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.

SEE ALSO

       mkswap(8), swapoff(8), swapon(8)

COLOPHON

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Linux man-pages 6.9.1          2024-05-02                      swapon(2)

Pages that refer to this page: syscalls(2), systemd.exec(5), capabilities(7), swapon(8)

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