swapon, swapoff — start/stop swapping to file/device
#include <unistd.h> #include <asm/page.h> /* to find PAGE_SIZE */ #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
.
swapon
() takes a swapflags
argument. If
swapflags
has the
SWAP_FLAG_PREFER
bit turned on,
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
These functions may only be used 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 non-negative 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
appropriately.
(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; 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 currently a
swap area.
The system 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.
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 kernel 2.6.10,
MAX_SWAPFILES
has the value 8;
since kernel 2.6.10, 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)).