Re: Can't mount subvolume with ro option

[Qu Wenruo <quwenruo@xxxxxxxxxxxxxx>]

-------- Original Message --------
Subject: Re: Can't mount subvolume with ro option
From: Qu Wenruo <quwenruo@xxxxxxxxxxxxxx>
To: Sébastien ROHAUT <sebastien.rohaut@xxxxxxx>, Chris Murphy 
<lists@xxxxxxxxxxxxxxxxx>
Date: 2014年06月30日 10:19
> -------- Original Message --------
> Subject: Re: Can't mount subvolume with ro option
> From: Sébastien ROHAUT <sebastien.rohaut@xxxxxxx>
> To: Chris Murphy <lists@xxxxxxxxxxxxxxxxx>
> Date: 2014年06月28日 19:02
> > Le 28/06/2014 00:12, Chris Murphy a écrit :
> > > On Jun 27, 2014, at 4:08 PM, Chris Murphy <lists@xxxxxxxxxxxxxxxxx> 
> > > wrote:
> > >
> > > > On Jun 27, 2014, at 2:07 PM, Sébastien ROHAUT 
> > > > <sebastien.rohaut@xxxxxxx> wrote:
> > > >
> > > > > Hi,
> > > > >
> > > > > In the wiki, it's said we can mount subvolumes with different 
> > > > > mount options. nosuid, nodev, rw and ro are listed, as valid 
> > > > > generic mount options.
> > > > This might require 3.15. I don't recall it working with early 3.14 
> > > > kernels, but by 3.14.3 I'd moved onto testing 3.15.
> > >
> > > [root@f20v ~]# mount /dev/sda3 /mnt
> > > [root@f20v ~]# btrfs subvol create /mnt/test
> > > Create subvolume '/mnt/test'
> > > [root@f20v ~]# umount /mnt
> > > [root@f20v ~]# mount -o ro,subvol=test /dev/sda3 /mnt
> > > mount: /dev/sda3 is already mounted or /mnt busy
> > >         /dev/sda3 is already mounted on /
> > >         /dev/sda3 is already mounted on /home
> > >         /dev/sda3 is already mounted on /var
> > >         /dev/sda3 is already mounted on /boot
> > > [root@f20v ~]# uname -r
> > > 3.14.6-200.fc20.x86_64
> > >
> > >
> > > I don't know if this feature will be backported to stable kernels. 
> > > If not, then probably the wiki should say it's a 3.15+ feature.
> > > Chris Murphy
> > Thank you for your answer. I'm actually moving to a Fedora Rawhide 
> > (3.16 kernel).
> >
> > Sebastien Rohaut
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> When testing on 3.15.2-ARCH alone with 3.16-rc2, the problem can also 
> be reproduced.
>
> But I can't always reproduce it, sometimes I was able to mount subvol 
> with ro,
> somtimes not.
>
> So I think the problem needs to be investigated further.
>
> Thanks,
> Qu
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Finally find the stable method to reproduce the problem on 3.16-rc2,
the point is if we mount subvol,ro first, then you can't mount the hole 
device:

# mkfs.btrfs  -f /dev/sda6
# mount /dev/sda6  /mnt/btrfs/
# btrfs sub create /mnt/btrfs/subv
Create subvolume '/mnt/btrfs/subv'
# umount /mnt/btrfs
# mount -o subvol=subv,ro /dev/sda6  /mnt/other/
# LANG=C mount /dev/sda6  /mnt/btrfs/
mount: /dev/sda6 is already mounted or /mnt/btrfs busy
       /dev/sda6 is already mounted on /mnt/other

Thanks,
Qu

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