linux-kernel - Re: Back to the future.

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Message-Id: <200704272244.08102.rjw@sisk.pl>
Date:	Fri, 27 Apr 2007 22:44:07 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Pekka J Enberg <penberg@...helsinki.fi>
Cc:	Nigel Cunningham <nigel@...el.suspend2.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Back to the future.

On Friday, 27 April 2007 06:52, 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.

The user space is (and IMHO should be) frozen way before that and what you're
suggesting here is what I wanted to implement some time ago.  The problem with
this was that the user space pages may be updated, for example, by device
drivers as a result of some deferred I/O after we've snapshotted the system.

I didn't know how to find out which pages owned by the user space could be
updated this way, so I gave up at that time.

Greetings,
Rafael

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