linux-kernel - Re: No /dev/root with devtmpfs?

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Message-ID: <4F32B2A0.70405@zytor.com>
Date:	Wed, 08 Feb 2012 09:36:32 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Kay Sievers <kay.sievers@...y.org>
CC:	Paul Parsons <lost.distance@...oo.com>,
	Kirill Smelkov <kirr@....spb.ru>, linux-kernel@...r.kernel.org
Subject: Re: No /dev/root with devtmpfs?

On 02/08/2012 09:25 AM, Kay Sievers wrote:
> On Wed, Feb 8, 2012 at 18:20, H. Peter Anvin <hpa@...or.com> wrote:
>> On 02/08/2012 08:44 AM, Paul Parsons wrote:
>>>
>>> Could you simply use /etc/fstab to identify the root partition?
>>>
>>
>> That's not a very good thing, as it is much more likely to be wrong.
>>
>> It would be a good thing to have the /dev/root symlink *IF* a valid root
>> device exists (defined as a device node appearing which has the same
>> device number as reported by stat on the root directory), if nothing
>> else because we have had one available for a very long time and this is
>> needless breakage.
>>
>> Obviously, if such a device doesn't exist (btrfs, NFS, tmpfs) then don't.
> 
> Tools should just do the equivalent of:
>   $ ls -l /sys/dev/block/$(mountpoint -d /)
>   /sys/dev/block/8:1 ->
> ../../devices/pci0000:00/0000:00:1f.2/host0/target0:0:0/0:0:0:0/block/sda/sda1
> 
> and all is fine. I'm convinced, that determining the root device is a
> job for *running code* not to expect a symlink to be around.
> 
> Kay

Okay, first of all, this is a service (you can think of it as such)
which has been provided for a long time.  It can be done with minimal
effort as devices appear, so you don't need to scan the /dev directory
to find the device node that corresponds to the above (as you know,
there are deliberately no device nodes in /sys, although *most* of the
time you obviously have the name right there.)

I'm not saying you're wrong, I'm saying you should consider it a
backwards compatibility solution for an interface which has been
provided for a long time already.

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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