linux-kernel - Re: [PATCH 2.6.19 5/5] fs: freeze

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Message-Id: <200611092059.48722.rjw@sisk.pl>
Date:	Thu, 9 Nov 2006 20:59:47 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Pavel Machek <pavel@....cz>
Cc:	Alasdair G Kergon <agk@...hat.com>,
	Eric Sandeen <sandeen@...hat.com>,
	Andrew Morton <akpm@...l.org>, linux-kernel@...r.kernel.org,
	dm-devel@...hat.com, Srinivasa DS <srinivasa@...ibm.com>,
	Nigel Cunningham <nigel@...pend2.net>,
	David Chinner <dgc@....com>
Subject: Re: [PATCH 2.6.19 5/5] fs: freeze_bdev with semaphore not mutex

Hi,

On Thursday, 9 November 2006 17:00, Pavel Machek wrote:
> Hi!
> 
> > > > Well, it looks like the interactions with dm add quite a bit of
> > > > complexity here.
> > > 
> > > What about just fixing xfs (thou shall not write to disk when kernel
> > > threads are frozen), and getting rid of blockdev freezing?
> > 
> > Well, first I must admit you were absolutely right being suspicious with
> > respect to this stuff.
> 
> (OTOH your patch found real bugs in suspend.c, so...)
> 
> > OTOH I have no idea _how_ we can tell xfs that the processes have been
> > frozen.  Should we introduce a global flag for that or something?
> 
> I guess XFS should just do all the writes from process context, and
> refuse any writing when its threads are frozen... I actually still
> believe it is doing the right thing, because you can't really write to
> disk from timer.

This is from a work queue, so in fact from a process context, but from
a process that is running with PF_NOFREEZE.

And I don't think we can forbid filesystems to use work queues.  IMO it's
a legitimate thing to do for an fs.

_But_.

Alasdair, do I think correctly that if there's a suspended device-mapper
device below an non-frozen filesystem, then sys_sync() would block just
as well as freeze_bdev() on this filesystem?

Rafael


-- 
You never change things by fighting the existing reality.
		R. Buckminster Fuller
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