overlayfs: "filesystem of lowerdir is not supported" on cdrom

overlayfs: "filesystem of lowerdir is not supported" on cdrom

[Tim Tassonis]

Hi

Just installed 3.18-rc2 and tried to test the overlayfs stuff:

$ mkdir /ovtmp
$ mount -t tmpfs tmpfs /ovtmp/
$ mkdir /ovtmp/work
$ mkdir /ovtmp/upper
$ mkdir /cdrw
$ mount /dev/sr0 /mnt
$ mount |egrep "ovtmp|sr0"

/dev/sr0 on /mnt type iso9660 (ro,relatime)
tmpfs on /ovtmp type tmpfs (rw,relatime)

$ mount -t overlayfs overlayfs \
 -olowerdir=/mnt,upperdir=/ovtmp/upper,workdir=/ovtmp/work /cdrw

mount: wrong fs type, bad option, bad superblock on overlayfs,
       missing codepage or helper program, or other error

       In some cases useful info is found in syslog - try
       dmesg | tail or so.

$ dmesg |tail

[ 3266.793218] overlayfs: filesystem of lowerdir is not supported

However, the doc in Documentation/filesystems/overlayfs.txt states:

"The lower filesystem can be any filesystem supported by Linux and does
not need to be writable.  The lower filesystem can even be another
overlayfs.  The upper filesystem will normally be writable and if it
is it must support the creation of trusted.* extended attributes, and
must provide valid d_type in readdir responses, so NFS is not suitable."

So: Is the documentation wrong, the error message in dmesg wrong, or
have missed something completely?


Kind regards
Tim

Re: overlayfs: "filesystem of lowerdir is not supported" on cdrom

[Al Viro]

On Tue, Oct 28, 2014 at 07:10:36PM +0100, Tim Tassonis wrote:
> Hi
> 
> Just installed 3.18-rc2 and tried to test the overlayfs stuff:
> 
> $ mkdir /ovtmp
> $ mount -t tmpfs tmpfs /ovtmp/
> $ mkdir /ovtmp/work
> $ mkdir /ovtmp/upper
> $ mkdir /cdrw
> $ mount /dev/sr0 /mnt
> $ mount |egrep "ovtmp|sr0"
> 
> /dev/sr0 on /mnt type iso9660 (ro,relatime)
> tmpfs on /ovtmp type tmpfs (rw,relatime)
> 
> $ mount -t overlayfs overlayfs \
>  -olowerdir=/mnt,upperdir=/ovtmp/upper,workdir=/ovtmp/work /cdrw

> [ 3266.793218] overlayfs: filesystem of lowerdir is not supported
> 
> However, the doc in Documentation/filesystems/overlayfs.txt states:
> 
> "The lower filesystem can be any filesystem supported by Linux and does
> not need to be writable.  The lower filesystem can even be another
> overlayfs.  The upper filesystem will normally be writable and if it
> is it must support the creation of trusted.* extended attributes, and
> must provide valid d_type in readdir responses, so NFS is not suitable."
> 
> So: Is the documentation wrong, the error message in dmesg wrong, or
> have missed something completely?

Documentation is wildly over-optimistic.  You are trying to use something
with non-trivial ->d_hash and ->d_compare as lower layer.

What should happen if you ask to mount an image with joliet extensions and
ask to create foo.txt.... when foo.txt is there?  From the isofs POV those
are the same; for overlayfs they are not (it's using default ->d_hash
and ->d_compare).

We probably ought to split the normal (case-sensitive, no joliet shite) case
out and leave it with NULL ->s_d_op, but that'll need to be done carefully,
or isofs_cmp() will blow up on us.

Re: overlayfs: "filesystem of lowerdir is not supported" on cdrom

[Al Viro]

On Tue, Oct 28, 2014 at 09:13:13PM +0000, Al Viro wrote:

> Documentation is wildly over-optimistic.  You are trying to use something
> with non-trivial ->d_hash and ->d_compare as lower layer.
> 
> What should happen if you ask to mount an image with joliet extensions and
> ask to create foo.txt.... when foo.txt is there?  From the isofs POV those
> are the same; for overlayfs they are not (it's using default ->d_hash
> and ->d_compare).
> 
> We probably ought to split the normal (case-sensitive, no joliet shite) case
> out and leave it with NULL ->s_d_op, but that'll need to be done carefully,
> or isofs_cmp() will blow up on us.

See if vfs.git#for-linus works for you.

Re: overlayfs: "filesystem of lowerdir is not supported" on cdrom

[Tim Tassonis]

On 10/28/2014 11:54 PM, Al Viro wrote:
> On Tue, Oct 28, 2014 at 09:13:13PM +0000, Al Viro wrote:
> 
>>
>> We probably ought to split the normal (case-sensitive, no joliet shite) case
>> out and leave it with NULL ->s_d_op, but that'll need to be done carefully,
>> or isofs_cmp() will blow up on us.
> 
> See if vfs.git#for-linus works for you.
> 

Yes, works for me! Could mount the overlayfs, remove a file, add a file
and modify a file. Not the definive stress-test I guess, but it's a start...

Took only the following two commits:

f643ff550afbc82a2bc7026f4a6d64427e4fbc9

5b71ecfa78271d5c576f17156ed8a53981c1ecb

Kind regards
Tim