linux-kernel - Re: Back to the future.

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Message-ID: <Pine.LNX.4.64.0704270910190.18521@sbz-30.cs.Helsinki.FI>
Date:	Fri, 27 Apr 2007 09:18:35 +0300 (EEST)
From:	Pekka J Enberg <penberg@...helsinki.fi>
To:	Nigel Cunningham <nigel@...el.suspend2.net>
cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Back to the future.

On Fri, 27 Apr 2007, Nigel Cunningham wrote:
> 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?

No. The snapshot is just that. A snapshot in time. From kernel point of 
view, it doesn't matter one bit what when you did it or if the state has 
changed before you resume. It's up to userspace to make sure the user 
doesn't do real work while the snapshot is being written to disk and 
machine is shut down.

On Fri, 27 Apr 2007, Nigel Cunningham wrote:
> 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.

When the snapshot pages are COW, we will run out of memory if userspace 
writes to those pages too much. If userspace is blocked, say like 
displaying a "we are suspending" in X which blocks the user from using 
other programs that could generate new writes and mounting filesystems 
read-only, we don't need to worry about running out of memory.

				Pekka
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