linux-kernel - Re: [dm-devel] linux-next - WARNING: at fs/block_dev.c:824 bd_link_disk

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Message-ID: <20110113122701.GG16523@nb.net.home>
Date:	Thu, 13 Jan 2011 13:27:01 +0100
From:	Karel Zak <kzak@...hat.com>
To:	Milan Broz <mbroz@...hat.com>
Cc:	device-mapper development <dm-devel@...hat.com>,
	Tejun Heo <tj@...nel.org>,
	"Jun'ichi Nomura" <j-nomura@...jp.nec.com>,
	Valdis.Kletnieks@...edu, linux-kernel@...r.kernel.org,
	linux-raid@...r.kernel.org,
	Alexander Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel@...r.kernel.org
Subject: Re: [dm-devel] linux-next - WARNING: at fs/block_dev.c:824
	bd_link_disk_holder+0x92/0x1ac()

On Thu, Jan 13, 2011 at 12:26:14PM +0100, Milan Broz wrote:
> On 01/13/2011 12:06 PM, Tejun Heo wrote:
> > Urgh... gees, so there actually was a user using all that cruft.
> > Sorry about the breakage, I'll see how multiple symlinks can be
> > restored.  I'm curious why this was added at all tho.  What was the
> > rationalization?  It's not like two subsystems can share the same
> > block device so marking the currently owning subsystem should have
> > been enough at the block layer.  There is no reason for block devices
> > to present information which is of no use to itself.  All that's
> > necessary is "this is taken by dm or md for more information, query
> > those".  dm and md need their own conf/representation layer anyway.
> 
> I am not sure if I understand it correctly, but multiple holders/slaves
> links are very useful in userspace to present device dependences.
> 
> We just implemented lsblk command in util-linux which simple prints
> tree according to these links, so dependendes of DM/MD/whatever devices
> can be printed without using any system specific callbacks, just using
> sysfs. See http://karelzak.blogspot.com/2010/12/lsblk8.html

 We also use holders/slaves links in libblkid to evaluate dependencies
 between devices (since 2008).

> Whatever changes are needed, please keep this functionality, it can be useful.

 Definitely.

    Karel

-- 
 Karel Zak  <kzak@...hat.com>
 http://karelzak.blogspot.com
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