Re: [Devel] Re: [PATCH 0/9] OpenVZ kernel based checkpointing/restart

[Oren Laadan <orenl@cs.columbia.edu>]

Andrey Mirkin wrote:
> On Monday 20 October 2008 19:55 Dave Hansen wrote:
>> On Mon, 2008-10-20 at 16:14 +0400, Andrey Mirkin wrote:
>>> Right now my patchset (v2) provides an ability to checkpoint and restart
>>> a group of processes. The process of checkpointing and restart can be
>>> initiated from external process (not from the process which should be
>>> checkpointed).
>> Absolutely.  Oren's code does it this way to make for a smaller patch at
>> first.  The syscall takes a pid argument so it is surely expected to be
>> expanded upon later.
>>
>>> Also I think that all the restart job (including process forking) should
>>> be done in kernel, as in this case we will not depend on user space and
>>> will be more secure. This is also implemented in my patchset.
>> Do you think that this is an approach that Oren's patches are married
>> to, or is this a "feature" we can add on later?
> 
> Well, AFAICS from Oren's patch set his approach is oriented on process 
> creation in user space. I think we should choose right now what approach will 
> be used for process creation.

This is inaccurate.

I intentionally did not address how processes will be created, by
simply allowing either way to be added to the patch.

I do agree that we probably want to decide how to do it. However,
there is also room to allow for both approaches, in a compatible
way, should we wish to explore both.

> We have two options here: fork processes in kernel or fork them in user space.
> If process will be forked in user space, then there will be a gap when process 
> will be in user space and can be killed with received signal before entering 

Why do we care about it ?
Why is there a difference if it is killed before or after entering
the kernel (e.g. user aborted restart, or kernel OOM kicked in) ?

> kernel. Also we will need a functionolity to create processes with predefined 
> PID. I think it is not very good to provide such ability to user space. That 
> is why we prefer in OpenVZ to do all the job in kernel.

This is the weak side of creating the processes in user space -
that we need such an interface. Note, however, that we can
easily "hide" it inside the interface of the sys_restart() call,
and restrict how it may be used.

Oren.

> 
>> I don't care which patch set we end up sticking in the kernel.  I'm
>> trying to figure out which code we can more easily build upon in the
>> future.  The fact that Oren's or yours can't do certain little things
>> right now does not bother me.
>>
>> Honestly, I'm a little more confident that everyone can work with Oren
>> since he managed to get 7 revisions of his patch out and make some
>> pretty large changes while in the same time the OpenVZ patch was only
>> released twice.  I'm not sure what has changed in the OpenVZ patch
>> between releases, either.
> 
> That is my fault. I am working right now on my Ph.D, that is why my activity 
> is not very high. But now I hope I will have more time for that.
> 
>> Are there any reasons that you absolutely can not use the code Oren
>> posted?  Will it not fulfill your needs somehow?  If so, could you
>> please elaborate on how?
> 
> We have one major difference with Oren's code - how processes are created 
> during restr.
> Right now I'm trying to port kernel process creation on top of Oren's patches.
> I agree that working in collaboration will speed up merging of checkpointing 
> to mainstream.
> 
> Andrey
> 
> P.S.: Sorry for late reply, my mailer attached your e-mail to wrong thread.
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