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

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Message-ID: <20061109232438.GS30653@agk.surrey.redhat.com>
Date:	Thu, 9 Nov 2006 23:24:38 +0000
From:	Alasdair G Kergon <agk@...hat.com>
To:	Pavel Machek <pavel@....cz>
Cc:	"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

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.

I'm suggesting sysfs or something built on bd_claim might offer a mechanism 
for traversing the stack.

Alasdair
-- 
agk@...hat.com
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