linux-kernel - Re: [RFC][PATCH -mm] Freezer: Handle uninterruptible tasks

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Date:	Mon, 9 Jul 2007 10:45:56 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Jeremy Maitin-Shepard <jbms@....edu>
cc:	Pavel Machek <pavel@....cz>, "Rafael J. Wysocki" <rjw@...k.pl>,
	pm list <linux-pm@...ts.linux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Nigel Cunningham <nigel@...el.suspend2.net>,
	Oliver Neukum <oliver@...kum.org>,
	Miklos Szeredi <miklos@...redi.hu>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Matthew Garrett <mjg59@...f.ucam.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [RFC][PATCH -mm] Freezer: Handle uninterruptible tasks

On Mon, 9 Jul 2007, Jeremy Maitin-Shepard wrote:

> Pavel Machek <pavel@....cz> writes:
> 
> [snip]
> 
> > I don't know how to do that mechanism... but if we knew where to trap
> > filesystem writes, we could simply freeze at that point, and at that
> > point only, no?
> 
> Any operation at all that has an external effect must not occur after
> the snapshot is made; otherwise, there will be random hard-to-find
> corruptions and other problems occurring as a result.  Thus, for
> example, any writes (either directly or indirectly through e.g. a
> filesystem) to non-volatile storage, any network traffic, any
> communication with hardware like a printer must be prevented after the
> snapshot.

You have forgotten one critical point: The writes to save the snapshot 
image must be allowed.  That's what makes it really hard.

Alan Stern

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