linux-kernel - Re: Banning checkpoint (was: Re: What can OpenVZ do?)

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Date:	Tue, 24 Feb 2009 09:43:51 -0600
From:	"Serge E. Hallyn" <serue@...ibm.com>
To:	Dave Hansen <dave@...ux.vnet.ibm.com>
Cc:	Alexey Dobriyan <adobriyan@...il.com>, hpa@...or.com,
	linux-api@...r.kernel.org, containers@...ts.linux-foundation.org,
	Nathan Lynch <nathanl@...tin.ibm.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	tglx@...utronix.de, viro@...iv.linux.org.uk, mpm@...enic.com,
	Ingo Molnar <mingo@...e.hu>, torvalds@...ux-foundation.org,
	Andrew Morton <akpm@...ux-foundation.org>, xemul@...nvz.org
Subject: Re: Banning checkpoint (was: Re: What can OpenVZ do?)

Quoting Dave Hansen (dave@...ux.vnet.ibm.com):
> On Tue, 2009-02-24 at 07:47 +0300, Alexey Dobriyan wrote:
> > > I think what I posted is a decent compromise.  It gets you those
> > > warnings at runtime and is a one-way trip for any given process.  But,
> > > it does detect in certain cases (fork() and unshare(FILES)) when it is
> > > safe to make the trip back to the "I'm checkpointable" state again.
> > 
> > "Checkpointable" is not even per-process property.
> > 
> > Imagine, set of SAs (struct xfrm_state) and SPDs (struct xfrm_policy).
> > They are a) per-netns, b) persistent.
> > 
> > You can hook into socketcalls to mark process as uncheckpointable,
> > but since SAs and SPDs are persistent, original process already exited.
> > You're going to walk every process with same netns as SA adder and mark
> > it as uncheckpointable. Definitely doable, but ugly, isn't it?
> > 
> > Same for iptable rules.
> > 
> > "Checkpointable" is container property, OK?
> 
> Ideally, I completely agree.
> 
> But, we don't currently have a concept of a true container in the
> kernel.  Do you have any suggestions for any current objects that we
> could use in its place for a while?

I think the main point is that it makes the concept of marking a task as
uncheckpointable unworkable.  So at sys_checkpoint() time or when we cat
/proc/$$/checkpointable, we can check for all of the uncheckpointable
state of both $$ and its container (including whether $$ is a container
init).  But we can't expect that (to use Alexey's example) when one task
in a netns does a certain sys_socketcall, all tasks in the container
will be marked uncheckpointable.  Or at least we don't want to.

Which means task->uncheckpointable can't be the big stick which I think
you were hoping it would be.

-serge
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