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

[Andrey Mirkin <major@openvz.org>]

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.
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 
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.

> 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.