linux-kernel - Re: [patch 5/5] elf: Add support for loading ET

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Date:	Fri, 21 Oct 2011 22:36:28 +0400
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Pavel Emelyanov <xemul@...allels.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andrey Vagin <avagin@...allels.com>,
	James Bottomley <jbottomley@...allels.com>,
	Glauber Costa <glommer@...allels.com>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...e.hu>,
	Dave Hansen <dave@...ux.vnet.ibm.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Daniel Lezcano <dlezcano@...ibm.com>,
	Alexey Dobriyan <adobriyan@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>
Subject: Re: [patch 5/5] elf: Add support for loading ET_CKPT files

On Fri, Oct 21, 2011 at 11:26:10AM -0700, Tejun Heo wrote:
> Hello,
> 
> On Wed, Oct 19, 2011 at 11:56:32PM +0400, Cyrill Gorcunov wrote:
> > > I find that quite difficult to agree with.  We're talking about some
> > > minor additions to prctl against updating exec path to do something it
> > > was never designed to do + new binary format.
> > 
> > Tejun, regardless the mm_struct entries, what about mapping vdso pages at the
> > place they had on snapshot time? How would we handle them?
> 
> We discussed this elsewhere but just for the record.
> 

Yes ;)

> I haven't really looked into vdso but what the currently suggested
> implementation isn't correct.  vdso is there so that kernel can
> provide userspace code which may vary depending on kernel version and
> the actual machine it's running on, so both saving vdso and restoring
> verbatim and asking the kernel to map its vdso at certain address are
> wrong.
> 

It's fine for first iteration (or rather for first RFC where such details
exactly to be revealed and discussed).

> The checkpointer would need to remember the vdso symtab from the
> original process and then somehow build indirect jump table at the
> same address which redirects to the vdso of the machine the target
> process is being restored on.  I'm not sure about the details of the
> implementation but it could be something like "please allow me to
> write to this original vdso address and keep your vdso elsewhere".
> 

Original vdso address should remain as it was at checkpoint time but
you're right that we need kinda bridge between various kernels versions
I think.

> I think this comes back to why one-stop-solution-in-kernel is a bad
> idea for CR.  My impression is that when people say "that would
> require too many small API updatesa", it usually means that they
> haven't really thought about each necessary piece enough and just
> hacked something up lumpy and fuzzy on the edges.  The thing is that
> no matter how you lump them, those problems don't go away and
> attacking things in small understandable API pieces not only ensures
> the update itself makes sense and may be useful for others too but
> also forces people to actually think about each problem.
> 

	Cyrill
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