linux-kernel - Re: [PATCH] Driver Core: devtmpfs - kernel-maintained tmpfs-based /dev

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Message-ID: <20090810153943.GB7652@kroah.com>
Date:	Mon, 10 Aug 2009 08:39:43 -0700
From:	Greg KH <greg@...ah.com>
To:	David Dillow <dave@...dillows.org>
Cc:	Greg KH <gregkh@...e.de>, Andi Kleen <andi@...stfloor.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	linux-kernel@...r.kernel.org, Kay Sievers <kay.sievers@...y.org>,
	Jan Blunck <jblunck@...e.de>, Harald Hoyer <harald@...hat.com>,
	Scott James Remnant <scott@...ntu.com>
Subject: Re: [PATCH] Driver Core: devtmpfs - kernel-maintained tmpfs-based
 /dev

On Sat, Aug 08, 2009 at 07:07:56PM -0400, David Dillow wrote:
> On Thu, 2009-08-06 at 11:31 -0700, Greg KH wrote:
> > On Thu, Aug 06, 2009 at 12:20:58PM -0400, David Dillow wrote:
> > > If you use mount by label or UUID, you still need udev (or other tools)
> > > in the initrd to find the right disk, correct?
> > 
> > Yes, you would.
> > 
> > > And for distros that want to support that, does this really reduce the
> > > amount of code in the initrd?
> > 
> > Yes it does, see the code in the Novell Moblin images for an example of
> > how to use devtmpfs with udev for how to do this.
> 
> I followed up with Greg offline to make sure I was looking at the right
> thing, but the upshot is that Moblin does not use an initrd and devtmpfs
> does not in any way reduce the amount of code for those distros that
> want to support more than the root=/dev/blah syntax.

Like Scott points out, this option now allows distros to use blkid
directly instead of udev if they want to implement things like this
without udev.

> Given Eric and Arjan's numbers about the time it takes to populate /dev
> from /sys -- pointing the speed problem squarely at udev or the ruleset
> -- I don't see much of a win for devtmpfs other than avoiding a
> static /dev to make init=/bin/sh work.

That's a very big win right there, don't you think?  Also, embedded and
rescue disks also would like it for this very reason.

> You cannot rely on it for a hotplug disk such as USB, as it doesn't do
> the root=LABEL=usbroot that you'd want to do since the location is not
> stable.

Again, not all the world requires such labels.

thanks,

greg k-h
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