linux-kernel - Re: [xfs-masters] Re: freeze vs freezer

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Date:	Tue, 1 Jul 2008 08:21:28 +1000
From:	Dave Chinner <david@...morbit.com>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	Jeremy Fitzhardinge <jeremy@...p.org>, xfs-masters@....sgi.com,
	Elias Oltmanns <eo@...ensachen.de>,
	Henrique de Moraes Holschuh <hmh@....eng.br>,
	Kyle Moffett <mrmacman_g4@....com>,
	Matthew Garrett <mjg59@...f.ucam.org>,
	David Chinner <dgc@....com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Jens Axboe <jens.axboe@...cle.com>
Subject: Re: [xfs-masters] Re: freeze vs freezer

On Mon, Jun 30, 2008 at 11:00:43PM +0200, Rafael J. Wysocki wrote:
> On Monday, 30 of June 2008, Dave Chinner wrote:
> > On Sun, Jun 29, 2008 at 11:37:31PM -0700, Jeremy Fitzhardinge wrote:
> > > Dave Chinner wrote:
> > >> On Mon, Jun 30, 2008 at 01:22:47AM +0200, Rafael J. Wysocki wrote:
> > >>> Well, it seems we can handle this on the block layer level, by temporarily
> > >>> replacing the elevator with something that will selectively prevent fs I/O
> > >>> from reaching the layers below it.
> > >>
> > >> Why? What part of freeze_bdev() doesn't work for you?
> > >
> > > Well, my original problem - which is still an issue - is that a process  
> > > writing to a frozen XFS filesystem is stuck in D state, and therefore  
> > > cannot be frozen as part of suspend.
> 
> I thought we were talking about the post-freezer situation.
> 
> > Silly me - how could I forget the three headed monkey getting in
> > the way of our happy trip to beer island?
> > 
> > Seriously, though, how is stopping I/O in the elevator is going to
> > change that?
> 
> We can do that after creating the image and before we let devices run again.
> This way we won't need to worry about the freezer.

You're suggesting that you let processes trying to do I/O continue
until *after* the memory image is taken?  How is that going to work?
You've got to quiesce the filesystems totally *before* taking an image
of memory - it's the only way to guarantee that they are the
in-memory state and on disk state are consistent state on resume.

Don't re-invent the wheel - use the API we already have that does
exactly what needs to be done.

> > What do you do with a sync I/O (read or write)? The 
> > process is going to have to go to sleep somewhere in D state waiting
> > for that I/O to complete.  If you're going to intercept such
> > processes somewhere else to do something magic, then why not put
> > that magic in vfs_check_frozen()?
> 
> This might work too, but it would be nice to do something independent of the
> freezer, so that we can drop the freezer when we want and not when we are
> forced to.

vfs_check_frozen() is completely independent of the process freezer.

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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