From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755482Ab2BGKGF (ORCPT ); Tue, 7 Feb 2012 05:06:05 -0500 Received: from mail-gx0-f174.google.com ([209.85.161.174]:63493 "EHLO mail-gx0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755289Ab2BGKGC convert rfc822-to-8bit (ORCPT ); Tue, 7 Feb 2012 05:06:02 -0500 MIME-Version: 1.0 In-Reply-To: <20120207075553.GB8632@tugrik.mns.mnsspb.ru> References: <20120206111853.GA11506@tugrik.mns.mnsspb.ru> <20120207075553.GB8632@tugrik.mns.mnsspb.ru> From: Kay Sievers Date: Tue, 7 Feb 2012 11:05:42 +0100 Message-ID: Subject: Re: No /dev/root with devtmpfs? To: Kirill Smelkov Cc: linux-kernel@vger.kernel.org Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Feb 7, 2012 at 08:55, Kirill Smelkov wrote: > On Mon, Feb 06, 2012 at 08:45:24PM +0100, Kay Sievers wrote: >> On Mon, Feb 6, 2012 at 12:18, Kirill Smelkov 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