From mboxrd@z Thu Jan  1 00:00:00 1970
From: Daniel Drake <dsd@laptop.org>
Subject: Re: Remount root RO after the root dentry drops from the namespace
Date: Wed, 18 Nov 2009 12:38:32 +0000
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To: Miklos Szeredi <miklos@szeredi.hu>
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On Wed, 2009-11-18 at 12:50 +0100, Miklos Szeredi wrote:
> You can easily make a directory a root of a mount with
> 
>  mount --bind $DIR $DIR
> 
> In your example, add this before 5.

Thanks for the suggestion! And following on from that, we would do
"mount -o remount,ro /path/to/realroot" during shutdown
where /path/to/realroot is the bind mount that we created based on your
advice?

Unfortunately, that approach doesn't solve the problem. If you remount a
bind-mount as read only then the "real" underlying mount is unaffected.
See the code flow in do_remount() :

	if (flags & MS_BIND)
		err = change_mount_flags(path->mnt, flags);
	else
		err = do_remount_sb(sb, flags, data, 0);

Any other ideas?

Thanks,
Daniel