[Devel] [RFC][PATCH 0/2] CR: save/restore a single, simple task

[Oren Laadan]

In the recent mini-summit at OLS 2008 and the following days it was
agreed to tackle the checkpoint/restart (CR) by beginning with a very
simple case: save and restore a single task, with simple memory
layout, disregarding other task state such as files, signals etc.

Following these discussions I coded a prototype that can do exactly
that, as a starter. This code adds two system calls - sys_checkpoint
and sys_restart - that a task can call to save and restore its state
respectively. It also demonstrates how the checkpoint image file can
be formatted, as well as show its nested nature (e.g. cr_write_mm()
-> cr_write_vma() nesting).

The state that is saved/restored is the following:
* some of the task_struct
* some of the thread_struct and thread_info
* the cpu state (including FPU)
* the memory address space

[The patch is against commit fb2e405fc1fc8b20d9c78eaa1c7fd5a297efde43
of Linus's tree (uhhh.. don't ask why), but against tonight's head too].

In the current code, sys_checkpoint will checkpoint the current task,
although the logic exists to checkpoint other tasks (not in the
checkpointee's execution context). A simple loop will extend this to
handle multiple processes. sys_restart restarts the current tasks, and
with multiple tasks each task will call the syscall independently.
(Actually, to checkpoint outside the context of a task, it is also
necessary to also handle restart-block logic when saving/restoring the
thread data).

It takes longer to describe what isn't implemented or supported by
this prototype ... basically everything that isn't as simple as the
above.

As for containers - since we still don't have a representation for a
container, this patch has no notion of a container. The tests for
consistent namespaces (and isolation) are also omitted.

Below are two example programs: one uses checkpoint (called ckpt) and
one uses restart (called rstr). Execute like this (as a superuser):

orenl:~/test$ ./ckpt > out.1
hello, world!  (ret=1)		<-- sys_checkpoint returns positive id
 				<-- ctrl-c
orenl:~/test$ ./ckpt > out.2
hello, world!  (ret=2)
 				<-- ctrl-c
orenl:~/test$ ./rstr < out.1
hello, world!  (ret=0)		<-- sys_restart return 0

(if you check the output of ps, you'll see that "rstr" changed its
name to "ckpt", as expected).

Hoping this will accelerate the discussion. Comments are welcome.
Let the fun begin :)

Oren.


============================== ckpt.c ================================

#define _GNU_SOURCE        /* or _BSD_SOURCE or _SVID_SOURCE */

#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <fcntl.h>
#include <unistd.h>
#include <asm/unistd_32.h>
#include <sys/syscall.h>

int main(int argc, char *argv[])
{
 	pid_t pid = getpid();
 	int ret;

 	ret = syscall(__NR_checkpoint, pid, STDOUT_FILENO, 0);
 	if (ret < 0)
 		perror("checkpoint");

 	fprintf(stderr, "hello, world!  (ret=%d)\n", ret);

 	while (1)
 		;

 	return 0;
}

============================== rstr.c ================================

#define _GNU_SOURCE        /* or _BSD_SOURCE or _SVID_SOURCE */

#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <fcntl.h>
#include <unistd.h>
#include <asm/unistd_32.h>
#include <sys/syscall.h>

int main(int argc, char *argv[])
{
 	pid_t pid = getpid();
 	int ret;

 	ret = syscall(__NR_restart, pid, STDIN_FILENO, 0);
 	if (ret < 0)
 		perror("restart");

 	printf("should not reach here !\n");

 	return 0;
}
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