[QUESTION] Why overlayfs cannot mounted as nfs_export and metacopy?

[Zhihao Cheng <chengzhihao1@xxxxxxxxxx>]

Hi, all.

As title said. I wonder to know the reason for overlayfs mount failure 
with '-o nfs_export=on,metacopy=on'.

I modified kernel to enable these two options 'on',  it looks like that 
overlayfs can still work fine under nfs_v4.

Besides, I can get no more information about the reason from source 
code, maybe I missed something.