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

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Date:	Thu, 13 Jan 2011 16:59:55 +0100
From:	Karel Zak <kzak@...hat.com>
To:	Kay Sievers <kay.sievers@...y.org>
Cc:	Tejun Heo <tj@...nel.org>, Milan Broz <mbroz@...hat.com>,
	device-mapper development <dm-devel@...hat.com>,
	"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 03:43:38PM +0100, Kay Sievers wrote:
> On Thu, Jan 13, 2011 at 15:30, Tejun Heo <tj@...nel.org> wrote:
> > On Thu, Jan 13, 2011 at 3:25 PM, Milan Broz <mbroz@...hat.com> wrote:
> >> Maybe, but this was not invented in DM/MD camp:-)
> >> Probably Kay or Greg can answer why it was done this way?
> 
> It's not from Greg or Kay. It just appeared some day in the context of dm. :)
> 
> And yes, symlinks *look* nice and simple for the outside, but they are
> not, and have all sorts of problems like non-atomic updates, make it

 Sounds like sysfs implementation problem, right?

 If there is noway to fix sysfs then we can add a generic ioctl or
 /sys/block/<device>/{slave,holder}_list files with list of
 holders/slaves.
 
 But please, don't force userspace to use *claimer-specific*
 methods to answer *generic questions* like slave/holder dependencies
 between devices.
 
> impossible to ever rename a device (as long as they copy the device
> name), and and and .... we should not add more of this.
> 
> >> If btrfs internally creates some virtual _block_ device for its pool, it should
> >> present it here too with slaves/holders. If not, why it should create any links there?
> >
> > Yeah, that's the most bothering part for me.  The biggest customers of
> > bd_claim are filesystems and all these custom symlinkeries don't do
> > nothing for them.  It just doesn't make a whole lot of sense to me.
> 
> Btrfs does not use any blockdev as the master for good reason, and it
> can never map its slaves inside of /sys/block. 

 Yep, expected and correct response :-)

    Karel

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