linux-kernel - Re: RFC: permit link(2) to work across --bind mounts ?

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Date:	Fri, 28 Dec 2007 22:02:51 -0800 (PST)
From:	dean gaudet <dean@...tic.org>
To:	Jan Engelhardt <jengelh@...putergmbh.de>
cc:	David Newall <david@...idnewall.com>, Mark Lord <lkml@....ca>,
	Al Viro <viro@....linux.org.uk>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: RFC: permit link(2)  to work across --bind mounts ?

On Sat, 29 Dec 2007, Jan Engelhardt wrote:

> 
> On Dec 28 2007 18:53, dean gaudet wrote:
> >p.s. in retrospect i probably could have arranged it more like this:
> >
> >  mount /dev/md1 $tmpmntpoint
> >  mount --bind $tmpmntpoint/var /var
> >  mount --bind $tmpmntpoint/home /home
> >  umount $tmpmntpoint
> >
> >except i can't easily specify that in fstab... and neither of the bind 
> >mounts would show up in df(1).  seems like it wouldn't be hard to support 
> >this type of subtree mount though.  mount(8) could support a single 
> >subtree mount using this technique but the second subtree mount attempt 
> >would fail because you can't temporarily remount the device because the 
> >mount point is gone.
> 
> Why is it gone?
> 
> mount /dev/md1 /tmpmnt
> mount --bind /tmpmnt/var /var
> mount --bind /tmpmnt/home /home
> 
> Is perfectly fine, and /tmpmnt is still alive and mounted. Additionally,
> you can
> 
> umount /tmpmnt
> 
> now, which leaves only /var and /home.

i was trying to come up with a userland-only change in mount(8) which
would behave like so:

# mount --subtree var /dev/md1 /var
  internally mount does:
  - mount /dev/md1 /tmpmnt
  - mount --bind /tmpmnt/var /var
  - umount /tmpmnt

# mount --subtree home /dev/md1 /home
  internally mount does:
  - mount /dev/md1 /tmpmnt
  - mount --bind /tmpmnt/home /home
  - umount /tmpmnt

but that second mount would fail because /dev/md1 is already mounted
(but the mount point is gone)...

it certainly works if i issue the commands individually as i described
-- but a change within mount(8) would have the benefit of working with
/etc/fstab too.

-dean
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