linux-kernel - Re: [PATCH 10/30] cr: core stuff

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Message-ID: <20090414152951.GA7703@us.ibm.com>
Date:	Tue, 14 Apr 2009 10:29:51 -0500
From:	"Serge E. Hallyn" <serue@...ibm.com>
To:	Oren Laadan <orenl@...columbia.edu>
Cc:	Alexey Dobriyan <adobriyan@...il.com>, akpm@...ux-foundation.org,
	containers@...ts.linux-foundation.org, xemul@...allels.com,
	dave@...ux.vnet.ibm.com, mingo@...e.hu, hch@...radead.org,
	torvalds@...ux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 10/30] cr: core stuff

Quoting Oren Laadan (orenl@...columbia.edu):
> 
> Hi,
> 
> Serge E. Hallyn wrote:
> > Quoting Alexey Dobriyan (adobriyan@...il.com):
> > 
> > Hi Alexey,
> > 
> > as far as I can see, the main differences between this patch and the
> > equivalent in Oren's tree are:
> > 
> > 1. kernel auto-selects container init to freeze
> 
> Actually, this eliminates the possibility to checkpoint a subtree of
> tasks, which (under some obvious constraints) can be a handy feature.

Yes, I agree.  As Dave pointed out on irc yesterday, this patch shows a
very definate whole-container-only point of view which is worth
discussing.

> > 2. kernel freezes tasks
> 
> IMHO better to do it in userspace - that way userspace can accomplish
> other tasks while tasks are frozen, such as snapshot the filesystem,
> or block/unblock the network.

That's a good point.

> Is there a good argument to do it kernel ?

Convenience?  I guess you don't have to worry about getting your
checkpoint job into a cgroup by itself ahead of time.

> > 3. no objhash taking references
> > 4. no hbuf
> > 5. always require CAP_SYS_ADMIN
> 
> I'm now convinced (thanks, Serge!) that it's better not to require
> this unless we strictly have to.

:)  Cool.

I think the perceived need for it comes, as above, from the pure
checkpoint-a-whole-container-only view.  So long as you will
checkpoint/restore a whole container, then you'll end up doing
something requiring privilege anyway.  But that is not all of
the use cases.

-serge
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