linux-kernel - Re: loop subsystem corrupted after mounting multiple btrfs sub-volumes

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Message-ID: <20160226210302.GE17997@ZenIV.linux.org.uk>
Date:	Fri, 26 Feb 2016 21:03:02 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Stanislav Brabec <sbrabec@...e.cz>
Cc:	"Austin S. Hemmelgarn" <ahferroin7@...il.com>,
	linux-kernel@...r.kernel.org, Jens Axboe <axboe@...nel.dk>,
	Btrfs BTRFS <linux-btrfs@...r.kernel.org>,
	David Sterba <dsterba@...e.cz>
Subject: Re: loop subsystem corrupted after mounting multiple btrfs
 sub-volumes

On Fri, Feb 26, 2016 at 09:37:22PM +0100, Stanislav Brabec wrote:

> Do I understand, that you are saying:
> 
> Yes, mounting multiple loop devices associated with one file is a
> legitimate use, but mount(8) should never do it, because it has other
> ugly side effects?

It's on the same level as "hey, let's have an nbd daemon run in qemu
guest, exporting a host file over nbd, import it to host as /dev/nbd69,
set a loopback device over the underlying file as /dev/loop42 and
ask e.g. xfs to recognize that it's dealing with the same underlying array
of bytes in both cases - wouldn't it be neat if it could do that?"

There's no magic.  Really.  Unexpected sharing of backing store between
apparently unrelated devices can cause trouble.  And I'm not sure how
to deal with -o loop in a sane way, TBH - automagical losetup is bloody
hard to get right.  Keep in mind that loop-over-loop is also possible...