linux-kernel - Re: [BUG] remount ro on loopback mount leaves unmountable filesystem behind

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Message-ID: <20080513225558.GU23843@nb.net.home>
Date:	Wed, 14 May 2008 00:55:58 +0200
From:	Karel Zak <kzak@...hat.com>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	David Chinner <dgc@....com>, lkml <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	util-linux-ng@...r.kernel.org
Subject: Re: [BUG] remount ro on loopback mount leaves unmountable
	filesystem behind

On Tue, May 13, 2008 at 06:23:40AM -0400, Christoph Hellwig wrote:
> On Tue, May 13, 2008 at 06:50:47PM +1000, David Chinner wrote:
> > Same thing, but remounting the loop filesystem readonly
> > before unmounting:
> > 
> > $ mount -t xfs /dev/sdb6 /mnt/scratch
> > $ mkdir /mnt/scratch/mnt
> > $ touch /mnt/scratch/img
> > $ mkfs.xfs -f -d file,name=/mnt/scratch/img,size=1g
> > $ mount -t xfs -o loop /mnt/scratch/img /mnt/scratch/mnt
> > $ mount -t xfs -o remount,ro /mnt/scratch/img /mnt/scratch/mnt
> > $ umount /mnt/scratch/mnt
> > $ umount /mnt/scratch
> > umount: /mnt/scratch: device is busy
> > umount: /mnt/scratch: device is busy
> 
> This is a problem in mount, no the kernel.  Before the remount the
> /etc mtab looks something like this:
> 
> /qemu/test.img /mnt xfs rw,loop=/dev/loop0 0 0
> 
> and after it looks something like this:
> 
> /qemu/test.img /mnt xfs ro 0 0

 Yes, that's mount(8) bug. It doesn't read old options from /etc/mtab
 when you remount by "mount -o remount <src> <dest>".

> As a workaround do a losetup -d /dev/loop0 after unmounting the
> filesystem.

 ... or use "mount -o remount <dest>", for example:

   mount -t xfs -o remount,ro /mnt/scratch/mnt


    Karel

-- 
 Karel Zak  <kzak@...hat.com>
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