Re: Removing shared subtrees?

From: Andy Lutomirski
Date: Mon Sep 29 2014 - 20:15:23 EST

On Mon, Sep 29, 2014 at 5:09 PM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
> On Mon, Sep 29, 2014 at 04:45:42PM -0700, Andy Lutomirski wrote:
>> As far as I know, shared subtrees in recursive bind mounts are a
>> misfeature that existed for the sole purpose of allowing recursive
>> binds + chroot to emulate mount namespaces.
> Wrong. Different namespaces vs. multiple mounts in the same namespace
> have nothing whatsoever with shared vs. slave. It's completely orthogonal.
>> But we have mount
>> namespaces, so what are they for?
> ???

No, really, what is this VFS feature for? It's a complicated,
confusing chunk of code. Why is it there?

>> They're totally fsked up. For example, don't try this on a live system:
>> # mount --make-rshared /
>> # mount --rbind / /mnt
>> # umount -l /mnt
>> It will unmount *everything*.
> So will umount -l /
>> On Fedora, you don't even need the
>> --make-rshared part. WTF?
> "Doctor, it hurts when I do it..."

I understand that:

# mount --make-rshared /
# mount --rbind / /mnt
# umount - /mnt/dev

should unmount /dev. That's the whole point. But why does unmounting
*/mnt* propagate like that? It doesn't unmount /. To me, this makes
about as much sense as having 'umount -l /mnt/dev' unmount /dev/pts
but *not* /dev would make.

> I can suggest a few more self-LARTs, if you are interested...
>> Can we just remove the feature entirely in linux-next and see if
>> anyone complains? I'm all for propagation across mount namespaces,
>> but I suspect that, at the very least, there is no legitimate reason
>> whatsoever for mounts to propagate from a recursive bind mount back to
>> the origin.
>> IOW, can we kill shared mounts and just keep private and slave mounts?
> What for?

Simplicity and comprehensibility.

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