From: "Austin S. Hemmelgarn" <ahferroin7@gmail.com>
To: Al Viro <viro@ZenIV.linux.org.uk>
Cc: Stanislav Brabec <sbrabec@suse.cz>,
linux-kernel@vger.kernel.org, Jens Axboe <axboe@kernel.dk>,
Btrfs BTRFS <linux-btrfs@vger.kernel.org>,
David Sterba <dsterba@suse.cz>
Subject: Re: loop subsystem corrupted after mounting multiple btrfs sub-volumes
Date: Fri, 26 Feb 2016 15:36:06 -0500 [thread overview]
Message-ID: <56D0B736.2050904@gmail.com> (raw)
In-Reply-To: <20160226203010.GD17997@ZenIV.linux.org.uk>
On 2016-02-26 15:30, Al Viro wrote:
> On Fri, Feb 26, 2016 at 03:05:27PM -0500, Austin S. Hemmelgarn wrote:
>>> Where is /mnt/2?
>> It's kind of interesting, but I can't reproduce _any_ of this
>> behavior with either ext4 or BTRFS when I manually set up the loop
>> devices and point mount(8) at those instead of using -o loop on a
>> file. That really seems to indicate that this is caused by something
>> mount(8) is doing when it's calling losetup. I'm running a mostly
>> unmodified version of 4.4.2 (the only modification that would come
>> even remotely close to this is that I changed the default mount
>> options for everything from relatime to noatime), and util-linux
>> 2.27.1 from Gentoo.
>
> Sigh... sys_mount() (mount_bdev(), actually) has no way to tell if two
> loop devices refer to the same underlying object. As far as it's
> concerned, you are asking to mount a completely unrelated block device.
> Which just happens to see the data (living in separate pagecache, even)
> modified behind its back (with some delay) after it gets written to another
> device. Filesystem drivers generally don't like when something is screwing
> the underlying data, to put it mildly...
>
> When you ask to mount the _same_ device, mount_bdev(), as well as btrfs
> counterpart, makes sure that you get a reference to the same struct
> super_block, which avoids all coherency problems - all mounted instances
> refer to the same in-core objects (dentries, inodes, page cache, etc.).
> They get separate struct vfsmount instances, but that only matters for
> mountpoint crossing.
>
> As soon as you've set the second /dev/loop alias for the same underlying
> file, you are asking for all kinds of trouble. If you use the same one
> consistently, you are OK. BTW, even
> losetup /dev/loop0 /dev/sda1
> mount -t ext2 /dev/sda1 /mnt/1
> mount -t ext2 /dev/loop0 /mnt/2
> is enough for trouble - you get (as far as ext2 knows) unrelated devices
> screwing each other, with no good way to predict that. And you need to
> check propagation through more than one layer - loop over loop over block
> is also possible.
>
> IMO on-demand losetup a-la -o loop is simply a bad idea...
>
I agree wholeheartedly and wasn't disputing any of this, I meant I'm not
seeing any of the odd mount(2) or /proc/self/mountinfo behavior that
Stanislav started the thread about. It was entirely trivial to get the
filesystem images I used into a state where they couldn't be mounted
again afterwards.
next prev parent reply other threads:[~2016-02-26 20:37 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-02-25 19:22 loop subsystem corrupted after mounting multiple btrfs sub-volumes Stanislav Brabec
2016-02-26 12:33 ` Austin S. Hemmelgarn
2016-02-26 15:50 ` Stanislav Brabec
2016-02-26 16:39 ` Austin S. Hemmelgarn
2016-02-26 17:07 ` Stanislav Brabec
2016-02-26 18:22 ` Austin S. Hemmelgarn
2016-02-26 19:31 ` Stanislav Brabec
2016-02-26 17:53 ` Al Viro
2016-02-26 19:12 ` Stanislav Brabec
2016-02-26 20:05 ` Austin S. Hemmelgarn
2016-02-26 20:30 ` Al Viro
2016-02-26 20:36 ` Austin S. Hemmelgarn [this message]
2016-02-26 21:00 ` Stanislav Brabec
2016-02-26 22:00 ` Valdis.Kletnieks
2016-02-29 14:56 ` Stanislav Brabec
2016-03-01 13:44 ` Ming Lei
2016-04-12 18:38 ` Stanislav Brabec
2016-02-26 20:37 ` Stanislav Brabec
2016-02-26 21:03 ` Al Viro
2016-02-26 21:36 ` Stanislav Brabec
2016-02-26 21:45 ` Al Viro
2016-02-29 13:11 ` Austin S. Hemmelgarn
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