linux-kernel - Re: Problem w/ CONFIG_DEBUG_BLOCK_EXT

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Message-ID: <48BD19B1.9040207@kernel.org>
Date:	Tue, 02 Sep 2008 12:47:13 +0200
From:	Tejun Heo <tj@...nel.org>
To:	"Alan D. Brunelle" <Alan.Brunelle@...com>
CC:	device-mapper development <dm-devel@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Jens Axboe <jens.axboe@...cle.com>
Subject: Re: Problem w/ CONFIG_DEBUG_BLOCK_EXT_DEVT

Hello,

Alan D. Brunelle wrote:
> I have found two problems in LVM2/DM w/ a potential new "experimental
> feature" in 2.6.28: CONFIG_DEBUG_BLOCK_EXT_DEVT (this is from Jens
> Axboe's origin/for-2.6.28 git branch)

It's a debug option and I don't expect it to be enabled in any
production kernel.

> "Conventionally, block device numbers are allocated from predetermined
> contiguous area.  However, extended block area may introduce
> non-contiguous block device numbers.  This option forces most block
> device numbers to be allocated from the extended space and spreads them
> to discover kernel or userland code paths which assume predetermined
> contiguous device number allocation."
> 
> W/ LVM2 & DM there are (at least) two issues:
> 
> (1) Device major numbers for some reason are /not/ being entered
> correctly into /proc/devices -- w/ CONFIG_DEBUG_BLOCK_EXT_DEVT=y I am
> seeing some devices w/ major "259" (a SATA controller) but no entry in
> /proc/devices. LVM2/DM will not find the entry in /proc/devices, and not
> allow any device w/ that major to be used with LVM commands.

Hmmm.. Adding a call to register_blkdev(), which will create the
corresponding entry in /proc/devices, isn't difficult at all but which
name would it use?  It'll be mix of block devices (hd and sds
currently).  If we introduce a new name there, say, ext-block, would
that work?  BTW, is there any specific reason why LVM2/DM can't use
/sys/block/* ?

> (2) Device minor numbers can be quite large, and the 10-character limits
> in dm/lib/libdm-deptree.c are too small.

Would it be difficult to increase that?

Thanks.

-- 
tejun
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