How to re-load all running applications from swap space into RAM?

The following quick-and-dirty python script dumps the memory of a process to stdout. This has the side effect of loading any swapped out page or mapped file. Call it as cat_proc_mem 123 456 789 where the arguments are process IDs.

This script is completely specific to Linux. It may be adaptable to other systems with a similar /proc structure (Solaris?), but forget about running it on e.g. *BSD. Even on Linux, you may need to change the definition of c_pid_t and the values of PTRACE_ATTACH and PTRACE_DETACH. This is a proof-of-principle script, not meant as an example of good programming practices. Use at your own risk.

Linux makes the memory of a process available as /proc/$pid/mem. Only certain address ranges are readable. These ranges can be found by reading the memory mapping information from the text file /proc/$pid/maps. The pseudo-file /proc/$pid/mem cannot be read by all processes that have the permission to read it: the reader process must have called ptrace(PTRACE_ATTACH, $pid).

#!/usr/bin/env python
import ctypes, re, sys

## Partial interface to ptrace(2), only for PTRACE_ATTACH and PTRACE_DETACH.
c_ptrace = ctypes.CDLL("libc.so.6").ptrace
c_pid_t = ctypes.c_int32 # This assumes pid_t is int32_t
c_ptrace.argtypes = [ctypes.c_int, c_pid_t, ctypes.c_void_p, ctypes.c_void_p]
def ptrace(attach, pid):
    op = ctypes.c_int(16 if attach else 17) #PTRACE_ATTACH or PTRACE_DETACH
    c_pid = c_pid_t(pid)
    null = ctypes.c_void_p()
    err = c_ptrace(op, c_pid, null, null)
    if err != 0: raise SysError, 'ptrace', err

## Parse a line in /proc/$pid/maps. Return the boundaries of the chunk
## the read permission character.
def maps_line_range(line):
    m = re.match(r'([0-9A-Fa-f]+)-([0-9A-Fa-f]+) ([-r])', line)
    return [int(m.group(1), 16), int(m.group(2), 16), m.group(3)]

## Dump the readable chunks of memory mapped by a process
def cat_proc_mem(pid):
    ## Apparently we need to ptrace(PTRACE_ATTACH, $pid) to read /proc/$pid/mem
    ptrace(True, int(pid))
    ## Read the memory maps to see what address ranges are readable
    maps_file = open("/proc/" + pid + "/maps", 'r')
    ranges = map(maps_line_range, maps_file.readlines())
    maps_file.close()
    ## Read the readable mapped ranges
    mem_file = open("/proc/" + pid + "/mem", 'r', 0)
    for r in ranges:
        if r[2] == 'r':
            mem_file.seek(r[0])
            chunk = mem_file.read(r[1] - r[0])
            print chunk,
    mem_file.close()
    ## Cleanup
    ptrace(False, int(pid))

if __name__ == "__main__":
    for pid in sys.argv[1:]:
        cat_proc_mem(pid)

See also more information on /proc/$pid/mem.

unswap () {
  cat_proc_mem "$@" >/dev/null
}

If you really have enough RAM available again you can use this sequence (as root):

$ swapoff -a
$ swapon -a

(to force the explicit swap-in of all your applications)

(assuming that you are using linux)

Just for completeness, GDB can dump process image. I didn't check that it unswaps it, but it has to---there's no other way to read the whole process memory:
gdb -p $mypid
followed by
(gdb) gcore /tmp/myprocess-core
Saved corefile /tmp/myprocess-core