From: "Rafael J. Wysocki" <rjw@sisk.pl>
To: Nigel Cunningham <ncunningham@linuxmail.org>
Subject: Re: [PATCH] Freeze bdevs when freezing processes.
Date: Thu, 26 Oct 2006 11:08:22 +0200
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Cc: David Chinner <dgc@sgi.com>, Pavel Machek <pavel@ucw.cz>,
       Andrew Morton <akpm@osdl.org>, LKML <linux-kernel@vger.kernel.org>,
       xfs@oss.sgi.com
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On Thursday, 26 October 2006 10:18, Nigel Cunningham wrote:
> Hi Dave.
> 
> On Thu, 2006-10-26 at 17:30 +1000, David Chinner wrote:
> > On Wed, Oct 25, 2006 at 09:05:56PM +0200, Rafael J. Wysocki wrote:
> > > On Wednesday, 25 October 2006 15:23, Nigel Cunningham wrote:
> > > > > 
> > > > > Well, my impression is that this is exactly what happens here: Something
> > > > > in the XFS code causes metadata to be written to disk _after_ the atomic
> > > > > snapshot.
> > > > > 
> > > > > That's why I asked if the dirty XFS metadata were flushed by a kernel thread.
> > > > 
> > > > When I first added bdev freezing it was because there was an XFS timer
> > > > doing writes.
> > > 
> > > Yes, I noticed you said that, but I'd like someone from the XFS team to either
> > > confirm or deny it.
> > 
> > We have daemons running in the background that can definitely do stuff
> > after a sync. hmm - one does try_to_freeze() after a wakeup, the
> > other does:
> > 
> >                 if (unlikely(freezing(current))) {
> >                         set_bit(XBT_FORCE_SLEEP, &target->bt_flags);
> >                         refrigerator();
> >                 } else {
> >                         clear_bit(XBT_FORCE_SLEEP, &target->bt_flags);
> >                 }
> > 
> > before it goes to sleep. So that one (xfsbufd - metadata buffer flushing)
> > can definitely wake up after the sync and do work, and the other could if
> > the kernel thread freeze occurs after the sync.
> > 
> > Another good question at this point - exactly how should we be putting
> > these thread to to sleep? Are both these valid methods for freezing them?
> > And should we be freezing when we wake up instead of before we go to
> > sleep? i.e. what are teh rules we are supposed to be following?
> 
> As you have them at the moment, the threads seem to be freezing fine.
> The issue I've seen in the past related not to threads but to timer
> based activity. Admittedly it was 2.6.14 when I last looked at it, but
> there used to be a possibility for XFS to submit I/O from a timer when
> the threads are frozen but the bdev isn't frozen.

Also there may be a problem if a workqueue is used for that, because
worker_threads run with PF_NOFREEZE set.

Greetings,
Rafael


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