Re: [RFC PATCH] Revert "btrfs: allow mounting btrfs subvolumes with different ro/rw options"

[Goffredo Baroncelli <kreijack@libero.it>]

On 07/08/2014 04:43 AM, Duncan wrote:
> The remaining problem to deal with is that if say the root subvol (id=5) 
> is mounted rw,subvolmode=rw, while a subvolume below it is mounted 
> subvolmode=ro, then what happens if someone tries to make an edit in the 
> portion of the filesystem visible in the subvolume, but from the parent, 
> id=5/root in this case?  Obviously if that modification is allowed from 
> the parent, it'll change what's visible in the child subvolume as well, 
> which would be rather unexpected.

The ro/rw status is a subvolume flag.
So if a subvolume is marked rw (or ro), is writable (not writable) in all
the mount(S)
This flag is not inheritable.

What could be strange is the following:

      # mount -o subvolid=5,rw /dev/sda1 /mnt/btrfs-root
      # btrfs subvol create /mnt/btrfs-root/subvolname/
then
      # touch /mnt/btrfs-root/subvolname/touch-file

succeeds; but

      # mount -o subvolid=5,rw /dev/sda1 /mnt/btrfs-root
      # btrfs subvol create /mnt/btrfs-root/subvolname/
      # mount -o subvol=subvolname,ro /dev/sda1 /mnt/btrfs-subvol
then
      # touch /mnt/btrfs-root/subvolname/touch-file2

fails.




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