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

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Date:	Tue, 1 Jul 2008 16:35:43 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Dave Chinner <david@...morbit.com>
Cc:	xfs-masters@....sgi.com, Jeremy Fitzhardinge <jeremy@...p.org>,
	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 Tuesday, 1 of July 2008, Dave Chinner wrote:
> On Tue, Jul 01, 2008 at 12:38:41AM +0200, Rafael J. Wysocki wrote:
> > On Tuesday, 1 of July 2008, Dave Chinner wrote:
> > > 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?
> > 
> > I'm not going to let the data get to the disk.
> 
> Yes, but you still haven't answered the original question - What are
> you going to do with sync I/O that leaves a process in D state
> because you've prevented the I/O from being completed?

I don't want to intercept those processes, just allow them to block on that I/O.

> > > > > 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.
> > 
> > Well, can you please tell me how exactly that works, then?
> 
> Try looking at the code. When we freeze a filesystem sb->s_frozen
> changes state depending on the level of freeze currently obtained
> by the filesystem. And:
> 
> #define vfs_check_frozen(sb, level) \
>         wait_event((sb)->s_wait_unfrozen, ((sb)->s_frozen < (level)))
> 
> Pretty bloody simple, really.

OK

Do all of the filesystems implement the freezing?

Rafael
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