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

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Message-ID: <CAPXgP10K0umgZco-T37nBsbnKznb6oP4e_k82af7sdvpPNeQJQ@mail.gmail.com>
Date:	Tue, 7 Feb 2012 11:05:42 +0100
From:	Kay Sievers <kay.sievers@...y.org>
To:	Kirill Smelkov <kirr@....spb.ru>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: No /dev/root with devtmpfs?

On Tue, Feb 7, 2012 at 08:55, Kirill Smelkov <kirr@....spb.ru> wrote:
> On Mon, Feb 06, 2012 at 08:45:24PM +0100, Kay Sievers wrote:
>> On Mon, Feb 6, 2012 at 12:18, Kirill Smelkov <kirr@....spb.ru> wrote:
>> > Recently I've reworked my system not to use udev, but use devtmpfs
>> > instead and discovered there is no /dev/root symlink in devtmpfs case.
>> >
>> > My setup uses /dev/root early to know what is the boot device and then
>> > do some operations on it like checksumming, etc...
>> >
>> > Now when /dev/root is gone the best workaround I could come up with is
>> > to   grep /proc/partitions for '[hs]d[a-z]1'  but it's ugly and will
>> > break when there are several block devices attached.
>> >
>> > Is it somehow possible to add /dev/root to devtmpfs?
>>
>> No, devtmpfs has no business in knowing anything about the rootfs or
>> who mounted what ans where. I can not create such links.
>>
>> The entire concept of /dev/root is flawed anyway, and nothing should
>> really depend on that.
>>
>> Modern filesystems will not offer a direct relation to a single block
>> device, they allocate an superblock which has a major == 0, so there
>> can be by definition never such a link. Better get rid of all uses of
>> /dev/root, it will just fail in the future.
>
> Kay, thanks for explanation.
>
> Though now I'm confused about how to know on what "place" root
> filesystem was mounted... To me the question makes sense, but you say
> with modern filesystems there by definition is no answer (or did I
> misunderstood you?) Strange...

The root filesystem will still be mounted at /, but you can not
identify by a device. Check the difference between:
  $ mountpoint -d /
  8:1
which is a classic ext4 sda1 device behind "/".

With 'btrfs' it will just look like 'tmpfs' looks today:
  $ mountpoint -d /dev/shm
  0:17

and while the 'btrfs' file system is surely backed by a
disk/partition, just like ext4 is, there can still never be a symlink
to the disk device.

Tools need to work without in the future without having a /dev/root symlink

Kay
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