linux-kernel - Re: [RFC][PATCH 8/8] check files for checkpointability

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Date:	Mon, 02 Mar 2009 10:13:20 -0800
From:	Dave Hansen <dave@...ux.vnet.ibm.com>
To:	"Serge E. Hallyn" <serue@...ibm.com>
Cc:	containers <containers@...ts.linux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	hch@...radead.org, Nathan Lynch <ntl@...ox.com>,
	Ingo Molnar <mingo@...e.hu>,
	Alexey Dobriyan <adobriyan@...il.com>
Subject: Re: [RFC][PATCH 8/8] check files for checkpointability

On Mon, 2009-03-02 at 11:44 -0600, Serge E. Hallyn wrote:
> Quoting Dave Hansen (dave@...ux.vnet.ibm.com):
> > On Mon, 2009-03-02 at 11:22 -0600, Nathan Lynch wrote:
> > > No.. I mean what if a process 1234 does
> > > 
> > > f = fopen("/proc/1234/stat", "r");
> > > 
> > > and is then checkpointed.  Can that path be resolved during restart,
> > > before pid 1234 is alive?
> > 
> > Heh, that's a good one.
> > 
> > It does mean that we can't do restore like this:
> > 
> > 	for_each_cr_task()
> > 		restore_task_struct()
> > 		restore_files()
> > 		...
> > 
> > We have to do:
> > 
> > 	for_each_cr_task()
> > 		restore_task_struct()
> > 	for_each_cr_task()
> > 		restore_files()
> >
> Which is what we actually do, right?

OK, I have a really evil one.

What if task 1234 does:

	open(O_RDONLY, "/proc/5678/fdinfo/44");

and task 5678 does:

	open(O_RDONLY, "/proc/5678/fdinfo/55");

There is no right order.

The only right way I can think to do it is that we have to loop on the
restore and defer files that we can't seem to find right now, hoping
that they'll show up as the restore progresses.

Basically:

	for_each_cr_task()
		deferred_files = restore_files()
retry:
	making_progress = 0
	for_each(deferred_file)
		restore(deferred_file)
	if (making_progress)
		goto retry;

-- Dave

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