Overlayfs with NFS lower layer is still not working

Hi,

I'm following the progress done for supporting NFS as lower layer.
It is still not working since ENXIO is returned on every attempt to open a file (linux 4.4 + overlayfs-next branch)

here is a simple example:

mkdir /tmp/nfs
mount -t nfs x.x.x.x:/export/foo -o nolock /tmp/nfs

mkdir -p /tmp/nv/overlay.wd
mkdir -p /tmp/nv/upper

mkdir /tmp/ovl
mount -t overlay overlay -olowerdir=/tmp/nfs,upperdir=/tmp/nv/upper,workdir=/tmp/nv/overlay.wd /tmp/ovl

# up to this point every thing is fine. "ls" is working. yet

cat /tmp/ovl/some_file_exisiting on /tmp/nfs
cat: can't open '/tmp/ovl/some_file_exisiting': No such device or address

I dig and I just found that it fails because every inode created by ovl_lookup() are keeping the initial no_open() (fs/inode.c) method as inode->i_fops->open,

[<800d38f4>] (inode_init_always) from [<800d4204>] (alloc_inode+0x48/0x98) [<800d4204>] (alloc_inode) from [<800d5fac>] (new_inode_pseudo+0x8/0x64) [<800d5fac>] (new_inode_pseudo) from [<800d6014>] (new_inode+0xc/0x20) [<800d6014>] (new_inode) from [<80179688>] (ovl_new_inode+0x10/0xe0) [<80179688>] (ovl_new_inode) from [<80178ae8>] (ovl_lookup+0x1d0/0x44c) [<80178ae8>] (ovl_lookup) from [<800c5f00>] (lookup_real+0x30/0x4c) [<800c5f00>] (lookup_real) from [<800c9030>] (lookup_open+0xd8/0x184) [<800c9030>] (lookup_open) from [<800c9284>] (do_last+0x1a8/0x6e8) [<800c9284>] (do_last) from [<800c9858>] (path_openat+0x94/0x134) [<800c9858>] (path_openat) from [<800caec8>] (do_filp_open+0x38/0x84) [<800caec8>] (do_filp_open) from [<800bd604>] (do_sys_open+0x124/0x1c4) [<800bd604>] (do_sys_open) from [<8000ed20>] (ret_fast_syscall+0x0/0x3c)

When the layer is not a remote one (eg. ramfs), d_select_inode() is called before the open() call, giving a change to change i_fops content with the real ones. But in NFS case, the i_fops switch done during d_select_inode() is not done, and vfs_open() will finally call no_open() returning ENXIO.

Just trying to also add a "d_select_inode" phase in "ovl_reval_dentry_operations" fix this... but the system crash later when the file is closed. Just to tell that there is still something missing...

Unfortunately, I'm definitely not familiar at all with VFS & co to understand exactly what is missing ;)

Regards,
Arnaud