[RFC][v7][PATCH 9/9]: Document clone2() syscall

[Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>]

Subject: [RFC][v7][PATCH 9/9]: Document clone2() syscall

This gives a brief overview of the clone2() system call.  We should
eventually describe more details in existing clone(2) man page or in
a new man page.

Changelog[v7]:
	- Rename clone_with_pids() to clone2()
	- Changes to reflect new prototype of clone2() (using clone_struct).

Signed-off-by: Sukadev Bhattiprolu <sukadev@vnet.linux.ibm.com>
---
 Documentation/clone2 |   85 +++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 85 insertions(+)

Index: linux-2.6/Documentation/clone2
===================================================================
--- /dev/null	1970-01-01 00:00:00.000000000 +0000
+++ linux-2.6/Documentation/clone2	2009-09-18 18:48:00.000000000 -0700
@@ -0,0 +1,85 @@
+
+struct clone_struct {
+	u64 flags;
+	u64 child_stack;
+	u32 nr_pids;
+	u32 parent_tid;
+	u32 child_tid;
+	u32 reserved1;
+	u64 reserved2;
+};
+
+clone2(struct clone_struct * __user clone_args, pid_t * __user pids)
+
+	In addition to doing everything that clone() system call does,
+	the clone2() system call:
+
+		- allows additional clone flags (all 32 bits in the flags
+		  parameter to clone() are in use)
+
+		- allows user to specify a pid for the child process in its
+		  active and ancestor pid name spaces.
+
+	This system call is meant to be used when restarting an application
+	from a checkpoint.  Such restart requires that the processes in the
+	application have the same pids they had when the application was
+	checkpointed. When containers are nested, the processes within the
+	containers exist in multiple pid namespaces and hence have multiple
+	pids to specify during restart.
+
+	The @pids defines the set of pids that should be assigned to the child
+	process in its active and ancestor pid name spaces. The descendant pid
+	namespaces do not matter since a process does not have a pid in
+	descendant namespaces, unless the process is in a new pid namespace
+	in which case the process is a container-init (and must have the pid 1
+	in that namespace).
+
+	See CLONE_NEWPID section of clone(2) man page for details about pid
+	namespaces.
+
+	The order pids in @pids corresponds to the nesting order of pid-
+	namespaces, with @pids[0] corresponding to the init_pid_ns.
+
+	If a pid in the @pids list is 0, the kernel will assign the next
+	available pid in the pid namespace, for the process.
+
+	If a pid in the @pids list is non-zero, the kernel tries to assign
+	the specified pid in that namespace.  If that pid is already in use
+	by another process, the system call fails with -EBUSY.
+
+	On success, the system call returns the pid of the child process in
+	the parent's active pid namespace.
+
+	On failure, clone2() returns -1 and sets 'errno' to one of following
+	values (the child process is not created).
+
+	EPERM	Caller does not have the SYS_ADMIN privilege needed to excute
+		this call.
+
+	EINVAL	The number of pids specified in 'clone_args.nr_pids' exceeds
+		the current nesting level of parent process
+
+	EBUSY	A requested pid is in use by another process in that name space.
+
+Example:
+
+	pid_t pids[] = { 77, 99 };
+	struct clone_struct cs;
+
+	cs.flags = (u64) SIGCHLD;
+	cs.child_stack = (u64) setup_child_stack();
+	cs.nr_pids = 2;
+	cs.parent_tid = 0;
+	cs.child_tid = 0;
+
+	rc = syscall(__NR_clone2, &cs, pids);
+
+	if (rc < 0) {
+		perror("clone2()");
+		exit(1);
+	} else if (rc) {
+		/* Parent */
+	} else {
+		/* Child */
+	}
+