[CRIU] [PATCH RFC] namespaces: use CLONE_VFORK with CLONE_VM when it is possible

[Pavel Emelyanov]

> @@ -1026,13 +1025,7 @@ int collect_user_ns(struct ns_id *ns, void *oarg)
>  		 * may do changes about CRIU's internal files states in memory,
>  		 * so pass CLONE_FILES to reflect that.
>  		 */
> -		stack_size = 2 * 1024 * 1024;
> -		stack = mmap(NULL, stack_size, PROT_WRITE | PROT_READ, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
> -		if (stack == MAP_FAILED) {
> -			pr_perror("Can't allocate stack");
> -			return -1;
> -		}
> -		pid = clone(dump_user_ns, stack + stack_size, CLONE_VM | CLONE_FILES | SIGCHLD, ns);
> +		pid = clone_noasan(dump_user_ns, CLONE_VM | CLONE_FILES | SIGCHLD | CLONE_VFORK, ns);

This seems legit. I has some worries about dump_user_ns() might screw things up in the
parent VM, but it seems not to.

So ... looks like ACK :) but CLONE_VM here itself is quite nasty :(

>  		if (pid == -1) {
>  			pr_perror("Can't clone");
>  			return -1;
> @@ -1045,7 +1038,6 @@ int collect_user_ns(struct ns_id *ns, void *oarg)
>  			pr_err("Can't dump nested user_ns: %x\n", status);
>  			return -1;
>  		}
> -		munmap(stack, stack_size);
>  		return 0;
>  	} else {
>  		if (__dump_user_ns(ns))
>