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

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Message-ID: <AANLkTi=LKYzrNsU7jc1Nr4W4CaQ_dmBGeQAquSCp_yAX@mail.gmail.com>
Date:	Thu, 13 Jan 2011 15:43:38 +0100
From:	Kay Sievers <kay.sievers@...y.org>
To:	Tejun Heo <tj@...nel.org>
Cc:	Milan Broz <mbroz@...hat.com>, Karel Zak <kzak@...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 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
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. Simple meta-blockdevs
like md/dm just don't fit into modern requirements of a filesystem
(directory snapshots, directory subvolumes, complex raids, hassle-free
resizing, ...)  -- hence btrfs is much more like a network-filesystem
mount than a stream of blocks like a disk, and does not fit at all
into this model.

Kay
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