linux-kernel - Re: [RFC][v8][PATCH 10/10]: Document clone3() syscall

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Date:	Wed, 21 Oct 2009 11:27:15 -0700
From:	Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com>
To:	Pavel Machek <pavel@....cz>
Cc:	linux-kernel@...r.kernel.org, Oren Laadan <orenl@...columbia.edu>,
	serue@...ibm.com, "Eric W. Biederman" <ebiederm@...ssion.com>,
	Alexey Dobriyan <adobriyan@...il.com>,
	Pavel Emelyanov <xemul@...nvz.org>,
	Andrew Morton <akpm@...l.org>, torvalds@...ux-foundation.org,
	mikew@...gle.com, mingo@...e.hu, hpa@...or.com,
	Nathan Lynch <nathanl@...tin.ibm.com>, arnd@...db.de,
	peterz@...radead.org, Louis.Rilling@...labs.com, roland@...hat.com,
	kosaki.motohiro@...fujitsu.com, randy.dunlap@...cle.com,
	linux-api@...r.kernel.org,
	Containers <containers@...ts.linux-foundation.org>,
	sukadev@...ibm.com
Subject: Re: [RFC][v8][PATCH 10/10]: Document clone3() syscall

Pavel Machek [pavel@....cz] wrote:
| Hi!
| 
| > This gives a brief overview of the clone3() system call.  We should
| 
| Thanks!
| 
| > eventually describe more details in existing clone(2) man page or in
| > a new man page.
| 
| M. Kerrisk (see MAINTAINERS) maintains man pages...

Ok. I copied linux-api and M. Kerrisk is looking at this patchset.

| 
| > Changelog[v8]:
| > 	- clone2() is already in use in IA64. Rename syscall to clone3()
| ...
| > Index: linux-2.6/Documentation/clone2
| > ===================================================================
| > --- /dev/null	1970-01-01 00:00:00.000000000 +0000
| > +++ linux-2.6/Documentation/clone2	2009-10-12 19:54:38.000000000 -0700
| 
| clone3?

Ah, yes, will fix.

| 
| > +struct clone_struct {
| > +	u64 flags;
| > +	u64 child_stack;
| 
| u64 seems wrong  on 32 bit platforms. ulong?
| 
| > +	u32 nr_pids;
| 
| So nr_pids is either 1 or 2?

No. With pid namespaces, which can be nested to arbitrary levels, each
process has several pid numbers - one in its own namespace and one in
each ancestor pid namespaces.

nr_pids specifies the number of pids the user cares about. IOW, if you
checkpoint a process that is 3 levels below the init-pid-ns, and you
care about the pids for those three levels, you would pass in an
array of 4 pid_ts and nr_pids would be 4.

| 
| > +	u32 reserved1;
| > +	u64 parent_tid;
| > +	u64 child_tid;
| > +	u64 reserved2;
| > +};
| > +
| 
| 
| > +	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.
| 
| Ok, so I'm confused.

Hope the above note on pid-namespaces helps, if not let me know.

| 
| > +	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 failure, clone3() returns -1 and sets 'errno' to one of following
| > +	values (the child process is not created).
| 
| Inconsistent with above. Syscalls really return -ERRCODE, errno is
| glibc magic.

Ok.  Will make it consistent to say that it returns one of the error codes.
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