Message-ID: <49B15F35.2010909@free.fr>
Date: Fri, 06 Mar 2009 18:36:53 +0100
From: Cedric Le Goater <legoater@free.fr>
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To: "Serge E. Hallyn" <serue@us.ibm.com>
CC: Greg Kurz <gkurz@fr.ibm.com>,
       containers <containers@lists.linux-foundation.org>,
       "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
       Dave Hansen <dave@linux.vnet.ibm.com>,
       Christoph Hellwig <hch@infradead.org>, Ingo Molnar <mingo@elte.hu>,
       Alexey Dobriyan <adobriyan@gmail.com>
Subject: Re: [RFC][PATCH 00/11] track files for checkpointability
References: <20090305163857.0C18F3FD@kernel>	<20090305174037.GA2274@x200.localdomain>	<1236280567.22399.99.camel@nimitz>	<20090305210840.GA2499@x200.localdomain>	<1236288427.22399.122.camel@nimitz>	<20090305220044.GA2819@x200.localdomain>	<1236352121.5732.80.camel@bahia> <20090306153549.GA898@us.ibm.com>
In-Reply-To: <20090306153549.GA898@us.ibm.com>
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Serge E. Hallyn wrote:
> Quoting Greg Kurz (gkurz@fr.ibm.com):
>> On Fri, 2009-03-06 at 01:00 +0300, Alexey Dobriyan wrote:
>>> On Thu, Mar 05, 2009 at 01:27:07PM -0800, Dave Hansen wrote:
>>>>> Imagine, unsupported file is opened between userspace checks
>>>>> for /proc/*/checkpointable and /proc/*/fdinfo/*/checkpointable
>>>>> and whatever, you stil have to do all the checks inside checkpoint(2).
>>>> Alexey, we have two problems here.  I completely agree that we have to
>>>> do complete and thorough checks of each file descriptor at
>>>> sys_checkpoint().  Any checks made at other times should not be trusted.
>>>>
>>>> The other side is what Ingo has been asking for.  How do we *know* when
>>>> we are checkpointable *before* we call (and without calling)
>>> This "without calling checkpoint(2)" results in much complications
>>> as demonstrated.
>>>
>>> task_struct and file are not like other structures because they are exposed
>>> in /proc. For PROC_FS=n kernels, one can't even check.
>>>
>>> You can do checkpoint(2) without actual dump. You pass, you're most
>>> certainly checkpointable (with inevitable race condition in mind).
>>>
>> Ahhh thank you very much Alexey ! I wanted to explain this to Dave a few
>> monthes ago but I failed... probably because of my poor English skills.
>>
>> https://lists.linux-foundation.org/pipermail/containers/2008-October/013549.html
>>
>> Why would we add checking all over the place when it MUST be done on the
>> sys_checkpoint() path ? The checkpoint(2) dry-run is definitely the way
>> to go.
> 
> I'm sure Dave understood that this was possible :)
> 
> But what you and Alexey are proposing does not and cannot fullfill
> Ingo's requirement.

And if Ingo's requirement is fulfilled, would any C/R patchset be acceptable ?

C.  
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