linux-kernel - Re: [RFC v13][PATCH 00/14] Kernel based checkpoint/restart

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Date:	Thu, 12 Feb 2009 14:57:37 -0800
From:	Dave Hansen <dave@...ux.vnet.ibm.com>
To:	Matt Mackall <mpm@...enic.com>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	orenl@...columbia.edu, linux-api@...r.kernel.org,
	containers@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	torvalds@...ux-foundation.org, viro@...iv.linux.org.uk,
	hpa@...or.com, Thomas Gleixner <tglx@...utronix.de>,
	Cedric Le Goater <clg@...ibm.com>,
	Pavel Emelyanov <xemul@...nvz.org>,
	Alexey Dobriyan <adobriyan@...il.com>
Subject: Re: [RFC v13][PATCH 00/14] Kernel based checkpoint/restart

On Thu, 2009-02-12 at 13:30 -0600, Matt Mackall wrote:
> On Thu, 2009-02-12 at 10:11 -0800, Dave Hansen wrote:
...
> >  * Filesystem state
> >   * contents of files
> >   * mount tree for individual processes
> >  * flock
> >  * threads and sessions
> >  * CPU and NUMA affinity
> >  * sys_remap_file_pages()
> 
> I think the real questions is: where are the dragons hiding? Some of
> these are known to be hard. And some of them are critical checkpointing
> typical applications. If you have plans or theories for implementing all
> of the above, then great. But this list doesn't really give any sense of
> whether we should be scared of what lurks behind those doors.

This is probably a better question for people like Pavel, Alexey and
Cedric to answer.  

> Some of these things we probably don't have to care too much about. For
> instance, contents of files - these can legitimately change for a
> running process. Open TCP/IP sockets can legitimately get reset as well.
> But others are a bigger deal.

Legitimately, yes.  But, practically, these are things that we need to
handle because we want to make any checkpoint/restart as transparent as
possible.  Resetting people's network connections is not exactly illegal
but not very nice or transparent either.

> Also, what happens if I checkpoint a process in 2.6.30 and restore it in
> 2.6.31 which has an expanded idea of what should be restored? Do your
> file formats handle this sort of forward compatibility or am I
> restricted to one kernel?

In general, you're restricted to one kernel.  But, people have mentioned
that, if the formats change, we should be able to write in-userspace
converters for the checkpoint files.  

-- Dave

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