linux-kernel - Re: [PATCH] driver-core: devtmpfs - driver core maintained /dev tmpfs

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Message-ID: <9b2b86520905021322l31845d2dubad98d78380aa1d5@mail.gmail.com>
Date:	Sat, 2 May 2009 21:22:44 +0100
From:	Alan Jenkins <sourcejedi.lkml@...glemail.com>
To:	Kay Sievers <kay.sievers@...y.org>
Cc:	Christoph Hellwig <hch@...radead.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Greg KH <greg@...ah.com>, Jan Blunck <jblunck@...e.de>,
	linux-arch@...r.kernel.org, viro@...iv.linux.org.uk,
	torvalds@...l.org, akpm@...l.org, adam@...drasil.com
Subject: Re: [PATCH] driver-core: devtmpfs - driver core maintained /dev tmpfs

On 5/2/09, Kay Sievers <kay.sievers@...y.org> wrote:
> On Sat, May 2, 2009 at 09:16, Christoph Hellwig <hch@...radead.org> wrote:
>> >After the rootfs is mounted by the kernel, the
>>> populated tmpfs is mounted at /dev. In initramfs, it can be moved
>>> to the manually mounted root filesystem before /sbin/init is
>>> executed.
>>
>> That for example is something that is not acceptable.  We really don't
>> want the kernel to mess with the initial namespace in such a major way.
>
> There is nothing like "mess around", it's not mounted at all, until
> the kernel mounts the root filesystem at /, then devtmpfs is mounted
> the first time, and only if it's compiled in because you asked for it.
> Also, just try:
>   egrep 'mknod|create_dev' init/*.c
> and see what we currently do.
>
>> Counter-proposal:  Re-introduce a proper mini-devfs.  All nodes in there
>> are kernel-created and not changeable which sorts out that whole
>> mess of both drivers and userspace messing with tree topology we had
>> both in original devfs and this new devtmpfs.  Single-instance so it can
>> be
>> populated before it's actually mounted somewhere, that way the kernel
>> doesn't have to do any policy devicision on where it's mounted.
>
> That sounds worse than devtpfs, and does not help for most of the
> mentioned problems we are trying to solve here.

On a narrow issue: do you really object to moving the "mount dev -t
devfs2 /dev" into userspace (and therefore giving it a user-visible
name)??  That would address Cristophs particular objection about
"messing around" with the initial namespace.  It means I can be 100%
sure I can boot an old initramfs with this option enabled.  And it
gives a nice clean way for new initramfs' to test for this feature -
when they try to mount it, it fails.  It would seem to make for a
rather smoother migration path.

It shouldn't mean too many more LOC, you're already doing the "single
instance" thing.

Thanks
Alan
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