Date: Fri, 10 Oct 2008 10:46:14 +0200
From: Ingo Molnar <mingo@elte.hu>
To: Dave Hansen <dave@linux.vnet.ibm.com>
Cc: "Serge E. Hallyn" <serue@us.ibm.com>,
       containers@lists.linux-foundation.org, arnd@arndb.de,
       linux-kernel@vger.kernel.org, "Rafael J. Wysocki" <rjw@sisk.pl>,
       Arjan van de Ven <arjan@infradead.org>,
       Peter Zijlstra <a.p.zijlstra@chello.nl>
Subject: Re: [RFC][PATCH 2/2] first callers of process_deny_checkpoint()
Message-ID: <20081010084614.GA319@elte.hu>
References: <20081009190405.13A253CB@kernel> <20081009190406.1B257119@kernel> <20081009194350.GA31214@us.ibm.com> <1223585671.11830.40.camel@nimitz>
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* Dave Hansen <dave@linux.vnet.ibm.com> wrote:

> On Thu, 2008-10-09 at 14:43 -0500, Serge E. Hallyn wrote:
> > Hmm, I don't know too much about aio, but is it possible to succeed with
> > io_getevents if we didn't first do a submit?  It looks like the contexts
> > are looked up out of current->mm, so I don't think we need this call
> > here.
> > 
> > Otherwise, this is neat.
> 
> Good question.  I know nothing, either. :)
> 
> My thought was that any process *trying* to do aio stuff of any kind 
> is going to be really confused if it gets checkpointed.  Or, it might 
> try to submit an aio right after it checks the list of them.  I 
> thought it best to be cautious and say, if you screw with aio, no 
> checkpointing for you!

as long as there's total transparency and the transition from CR-capable 
to CR-disabled state is absolutely safe and race-free, that should be 
fine.

I expect users to quickly cause enough pressure to reduce the NOCR areas 
of the kernel significantly ;-)

In the long run, could we expect a (experimental) version of hibernation 
that would just use this checkpointing facility to hibernate? That would 
be way cool for users and for testing: we could do transparent kernel 
upgrades/downgrades via this form of hibernation, between CR-compatible 
kernels (!).

distros could mark certain kernels as 'safe fallback' kernels, and if 
say a WARN_ON() or app breakage hits that is suspected to be kernel 
related, the user could hit a 'switch back to safe kernel' button - 
which would switch back _without losing any app state_.

People could even try new versions of the kernel which would just fall 
back to the known-workin safe kernel if the bootup fails for example.

Pie in the sky for sure, but way cool: it could propel Linux kernel 
testing to completely new areas - new kernels could be tried 
non-intrusively. (as long as a new kernel does not corrupt the CR data 
structures - so some good consistency and redundancy checking would be 
nice in the format!)

	Ingo
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