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
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