linux-kernel - Re: [RFC v6][PATCH 0/9] Kernel based checkpoint/restart

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Date:	Mon, 13 Oct 2008 10:13:09 +0200
From:	Cedric Le Goater <clg@...ibm.com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Dave Hansen <dave@...ux.vnet.ibm.com>, jeremy@...p.org,
	arnd@...db.de, containers@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Alexander Viro <viro@...iv.linux.org.uk>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Oren Laadan <orenl@...columbia.edu>,
	Andrey Mirkin <major@...nvz.org>
Subject: Re: [RFC v6][PATCH 0/9] Kernel based checkpoint/restart

Ingo Molnar wrote:
> * Dave Hansen <dave@...ux.vnet.ibm.com> wrote:
> 
>> On Thu, 2008-10-09 at 15:44 +0200, Ingo Molnar wrote:
>>> there might be races as well, especially with proxy state - and 
>>> current->flags updates are not serialized.
>>>
>>> So maybe it should be a completely separate flag after all? Stick it 
>>> into the end of task_struct perhaps.
>> What do you mean by proxy state?  nsproxy?
> 
> it's a concept: one task installing some state into another task (which 
> state must be restored after a checkpoint event), while that other task 
> is running. Such as a pi-futex state for example.
> 
> So a task can acquire state not just by its own doing, but via some 
> other task too.

thinking aloud,

hmm, that's rather complex, because we have to take into account the 
kernel stack, no ? This is what Andrey was trying to solve in his patchset 
back in September :

	http://lkml.org/lkml/2008/9/3/96

the restart phase simulates a clone and switch_to to (not) restore the kernel 
stack. right ? 

the self checkpoint and self restore syscalls, like Oren is proposing, are 
simpler but they require the process cooperation to be triggered. we could
image doing that in a special signal handler which would allow us to jump
in the right task context. 

I don't have any preference but looking at the code of the different patchsets
there are some tricky areas and I'm wondering which path is easier, safer, 
and portable. 

C.

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