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

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Message-ID: <4D2EE156.1050006@redhat.com>
Date:	Thu, 13 Jan 2011 12:26:14 +0100
From:	Milan Broz <mbroz@...hat.com>
To:	device-mapper development <dm-devel@...hat.com>
CC:	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, Karel Zak <kzak@...hat.com>
Subject: Re: [dm-devel] linux-next - WARNING: at fs/block_dev.c:824 bd_link_disk_holder+0x92/0x1ac()

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

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

Thanks,
Milan
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