[Unionfs] Can't make branch read only

[Olivier Blin]

Erez Zadok <ezk at cs.sunysb.edu> writes:

>> - mount -t unionfs -o remount,mode=/live/foo=ro none /live/union
>> 
>> The last command, while should switch the branch to read only mode as the
>> documentation specifies, doesn't work, the branch /live/foo continues to be
>> read-write, and if you write something to /live/union you confirm this as there
>> will be still writes to /live/foo (and /proc/mounts continues to show /live/foo as
>> rw too). Any ideas?
>> 
>> I tried this with unionfs 2.1.3 and also with 2.1.7 but with same results.
>
> Herton, you cannot set the leftmost branch to be readonly, in a union
> mounted read-write.  If the union is mounted read-write, then the leftmost
> branch must be writeable, to allow copyup.  If you want to set the leftmost
> branch to readonly, it is the same as saying that you want the whole union
> to be readonly.  In that case, remount your entire filesystem readonly:
>
> # mount -t unionfs -o remount,ro none /live/union

Our use case is / as union, so we can't really remount the union
read-only (it's busy).

We used to be able to make the lefmost branch read-only with unionfs
1.x, even if the union was writable.
This allowed to boot with a tmpfs as rw branch of the / union, ask for
some settings, remount the tmpfs branch ro, safely copy the changes
from tmpfs to a new directory (on disk), and add this new directory in
the union with rw mode.

> Check your kernel console messages (e.g., run dmesg).  You'll should have
> seen this message:
>
> 	printk(KERN_ERR "unionfs: leftmost branch cannot be read-only "
> 		"(use \"remount,ro\" to create a read-only union)\n");

Actually, the "mount -t unionfs -o remount,mode=/live/foo=ro none /live/union"
command succeeds, with no error message, and nothing in syslog.

Thanks

-- 
Olivier Blin (blino) - Mandriva