linux-kernel - Removing shared subtrees?

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Date:	Mon, 29 Sep 2014 16:45:42 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Al Viro <viro@...iv.linux.org.uk>
Subject: Removing shared subtrees?

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.  But we have mount
namespaces, so what are they for?

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*.  On Fedora, you don't even need the
--make-rshared part.  WTF?

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?

--Andy

-- 
Andy Lutomirski
AMA Capital Management, LLC
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