Re: Back to the future.

From: Pekka J Enberg
Date: Fri Apr 27 2007 - 02:19:08 EST

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.

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