linux-kernel - Re: Back to the future.

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Message-Id: <1177654110.4737.91.camel@nigel.suspend2.net>
Date:	Fri, 27 Apr 2007 16:08:30 +1000
From:	Nigel Cunningham <nigel@...el.suspend2.net>
To:	Pekka J Enberg <penberg@...helsinki.fi>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Back to the future.

Hi.

On Fri, 2007-04-27 at 07:52 +0300, Pekka J Enberg wrote:
> On Thu, 2007-04-26 at 09:56 -0700, Linus Torvalds wrote:
> > >    which will map in the snapshot, return the mapped address and the size 
> > >    (and if you want to support snapshots > 4GB, be my guest, but I suspect 
> > >    you're actually *better* off just admitting that if you cannot shrink 
> > >    the snapshot to less than 32 bits, it's not worth doing)
>  
> On Fri, 27 Apr 2007, Nigel Cunningham wrote:
> > That inherently limits the image to half of available ram (you need
> > somewhere to store the snapshot), so you won't get the full image you
> > express interest in below.
> 
> It doesn't. We can make the userspace mapped pages copy-on-write. As long 
> as the userspace makes sure there's not much activity during 
> snapshot/shutdown, we will be fine. What we probably do need to copy is 
> kernel pages.

COW is a possibility, but I understood (perhaps wrongly) that Linus was
thinking of a single syscall or such like to prepare the snapshot. If
you're going to start doing things like this, won't that mean you'd then
have to update/redo the snapshot or somehow nullify the effect of
anything the programs does so that doing it again after the snapshot is
restored doesn't cause problems?

I was going to leave it at that and press send, but perhaps that
wouldn't be wise. I feel I should also ask what you're thinking of as a
means of making sure userspace doesn't do much activity.

Thanks for your labours!

Regards,

Nigel

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