linux-kernel - Re: [PATCH 18/38] C/R: core stuff

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Message-Id: <20090527154001.86a5abff.akpm@linux-foundation.org>
Date:	Wed, 27 May 2009 15:40:01 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Alexey Dobriyan <adobriyan@...il.com>
Cc:	orenl@...columbia.edu, serge@...lyn.com,
	linux-kernel@...r.kernel.org,
	containers@...ts.linux-foundation.org, xemul@...allels.com
Subject: Re: [PATCH 18/38] C/R: core stuff

On Thu, 28 May 2009 02:17:53 +0400
Alexey Dobriyan <adobriyan@...il.com> wrote:

> > 1) Whether or not allow c/r of sub-container (partial hierarchy)
> > 
> > 2) Creation of restarting process hierarchy in kernel or in userspace
> > 
> > As for #1, you are the _only_ one who advocates restricting c/r to
> > a full container only. I guess you have your reasons, but I'm unsure
> > what they may be.
> 
> The reason is that checkpointing half-frozen, half-live container is
> essentially equivalent to live container which adds much complexity
> to code fundamentally preventing kernel from taking coherent snapshot.
> 
> In such situations kernel will do its job badly.
> 
> Manpage will be filled with strings like "if $FOO is shared then $BAR is
> not guaranteed".
> 
> What to do if user simply doesn't know if container is bounded?
> Checkpoint and to hell with consequences?
> 
> If two tasks share mm_struct you can't even detect that pages you dump
> aren't filled with garbage meanwhile from second task.
> 
> If two tasks share mm_struct, other task can issue AIO indefinitely
> preventing from taking even coherent filesystem snapshot.
> 
> That's why I raise this issue again to hear from people what they think
> and these people shouldn't be containers and C/R people, because the
> latter already made up their minds.
> 
> This is super-important issue to get right from the beginning.

<pipes up>

yeah, checkpointing a partial hierarchy at this stage sounds like
overreach.  Get full-container working usably first, think about
sub-containers in version 2.

<pipes down again>
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