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       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


       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-
       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


       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.


       These functions are Linux-specific and should not be  used  in  programs  intended  to  be
       portable.  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
       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) 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.

       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)