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

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Message-ID: <20061109233258.GH2616@elf.ucw.cz>
Date:	Fri, 10 Nov 2006 00:32:58 +0100
From:	Pavel Machek <pavel@....cz>
To:	"Rafael J. Wysocki" <rjw@...k.pl>,
	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 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.

									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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