int swapon(const char *path, int swapflags);
int swapoff(const char *path);
flag is specified in the
argument, the new swap area will have a higher priority than default.
The priority is encoded within
(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).
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.
There is an upper limit on the number of swap files that may be used, defined by the kernel 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 CONFIG_MEMORY_FAILURE option.
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.