linux-kernel - Re: Banning checkpoint (was: Re: What can OpenVZ do?)

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Date:	Thu, 19 Feb 2009 11:11:54 -0800
From:	Dave Hansen <dave@...ux.vnet.ibm.com>
To:	Alexey Dobriyan <adobriyan@...il.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Nathan Lynch <nathanl@...tin.ibm.com>,
	linux-api@...r.kernel.org, containers@...ts.linux-foundation.org,
	mpm@...enic.com, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	viro@...iv.linux.org.uk, hpa@...or.com,
	Andrew Morton <akpm@...ux-foundation.org>,
	torvalds@...ux-foundation.org, tglx@...utronix.de, xemul@...nvz.org
Subject: Re: Banning checkpoint (was: Re: What can OpenVZ do?)

On Thu, 2009-02-19 at 22:06 +0300, Alexey Dobriyan wrote:
> Inotify isn't supported yet? You do
> 
>         if (!list_empty(&inode->inotify_watches))
>                 return -E;
> 
> without hooking into inotify syscalls.
> 
> ptrace(2) isn't supported -- look at struct task_struct::ptraced and
> friends.
> 
> And so on.
> 
> System call (or whatever) does something with some piece of kernel
> internals. We look at this "something" when walking data structures
> and
> abort if it's scary enough.
> 
> Please, show at least one counter-example.

Alexey, I agree with you here.  I've been fighting myself internally
about these two somewhat opposing approaches.  Of *course* we can
determine the "checkpointability" at sys_checkpoint() time by checking
all the various bits of state.

The problem that I think Ingo is trying to address here is that doing it
then makes it hard to figure out _when_ you went wrong.  That's the
single most critical piece of finding out how to go address it.

I see where you are coming from.  Ingo's suggestion has the *huge*
downside that we've got to go muck with a lot of generic code and hook
into all the things we don't support.

I think what I posted is a decent compromise.  It gets you those
warnings at runtime and is a one-way trip for any given process.  But,
it does detect in certain cases (fork() and unshare(FILES)) when it is
safe to make the trip back to the "I'm checkpointable" state again.

-- Dave

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