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

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Date:	Fri, 10 Nov 2006 13:03:32 +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

On Friday, 10 November 2006 00:32, Pavel Machek wrote:
> Hi!
> 
> > On Fri, Nov 10, 2006 at 12:11:46AM +0100, Pavel Machek wrote:
> > > ? Not sure if I quite understand, but if dm breaks sync... something
> > > is teribly wrong with dm. And we do simple sys_sync()... so I do not
> > > think we have a problem.
> >  
> > If you want to handle arbitrary kernel state, you might have a device-mapper
> > device somewhere lower down the stack of devices that is queueing any I/O
> > that reaches it.  So anything waiting for I/O completion will wait until 
> > the dm process that suspended that device has finished whatever it is doing
> > - and that might be a quick thing carried out by a userspace lvm tool, or
> > a long thing carried out by an administrator using dmsetup.
> > 
> > I'm guessing you need a way of detecting such state lower down the stack
> > then optionally either aborting the operation telling the user it can't be
> > done at present; waiting for however long it takes (perhaps for ever if
> > the admin disappeared); or more probably skipping those devices on a 
> > 'best endeavours' basis.
> 
> Okay, so you claim that sys_sync can stall, waiting for administator?
> 
> In such case we can simply do one sys_sync() before we start freezing
> userspace... or just more the only sys_sync() there. That way, admin
> has chance to unlock his system.

Well, this is a different story.

My point is that if we call sys_sync() _anyway_ before calling
freeze_filesystems(), then freeze_filesystems() is _safe_ (either the
sys_sync() blocks, or it doesn't in which case freeze_filesystems() won't
block either).

This means, however, that we can leave the patch as is (well, with the minor
fix I have already posted), for now, because it doesn't make things worse a
bit, but:
(a) it prevents xfs from being corrupted and
(b) it prevents journaling filesystems in general from replaying journals
after a failing resume.

Still, there is a problem with the possibility of potential lock-up - either
with the bdevs-freezing patch or without it - due to a suspended dm device
down the stack and solving that is a _separate_ issue.

Now if we use the userland suspend, there's no problem at all, I think,
because s2disk calls sync() before it goes to suspend_system(), so the
admin will have a chance to unclock the system and everything is fine and
dandy (although it should be documented somewhere, IMHO).

However, if the built-in swsusp is used, then well ... <looks> ... we can put
a call to sys_sync() before prepare_processes() in pm_suspend_disk().

Greetings,
Rafael

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