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

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Date:	Fri, 26 Feb 2016 22:00:44 +0100
From:	Stanislav Brabec <sbrabec@...e.cz>
To:	Al Viro <viro@...IV.linux.org.uk>,
	"Austin S. Hemmelgarn" <ahferroin7@...il.com>
Cc:	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 Feb 26, 2016 at 21:30 Al Viro wrote:
> IMO on-demand losetup a-la -o loop is simply a bad idea...

So the correct behavior of -o loop should:

Check, whether another mount command already did losetup.

If not, allocate new loop device.

If yes, reuse existing loop device.

Well, it seems to be safe, even if the loop device was not allocated by 
mount(8) itself, as
ioctl(fd, LOOP_CLR_FD)
never returns EBUSY:

# losetup /dev/loop2 /ext4.img
# mount /dev/loop2 /mnt
# strace losetup -d /dev/loop2 2>&1 | tail -n7 | head -n3
open("/dev/loop2", O_RDONLY|O_CLOEXEC)  = 3
ioctl(3, LOOP_CLR_FD)                   = 0
close(3)                                = 0

If the recycling "alien" loop devices will not be considered as a good 
idea, then (if possible):

If the loop device was allocated by mount(8) itself, recycle it.

If the loop device was not allocated by mount(8) itself, return error.

-- 
Best Regards / S pozdravem,

Stanislav Brabec
software developer
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