Support nfs server on ARM system

I am working on adding a nfs server on our arm embedded system.

1. recompile kernel to support it

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make menuconfig

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File systems -> Network File Systems

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NFS server support //highlight this one

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Then the config looks like that

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--- Network File Systems                                          
<*>   NFS client support                                             
[*]     NFS client support for NFS version 3                        
[ ]       NFS client support for the NFSv3 ACL protocol extension
[ ]     NFS client support for NFS version 4 (EXPERIMENTAL)
[*]     Root file system on NFS                            
<*>   NFS server support                                   
[*]     NFS server support for NFS version 3               
[ ]       NFS server support for the NFSv3 ACL protocol extension
[ ]     NFS server support for NFS version 4 (EXPERIMENTAL)       
< >   Secure RPC: Kerberos V mechanism (EXPERIMENTAL)            
< >   Secure RPC: SPKM3 mechanism (EXPERIMENTAL)                 
< >   SMB file system support (OBSOLETE, please use CIFS)        
< >   CIFS support (advanced network filesystem, SMBFS successor)
< >   NCP file system support (to mount NetWare volumes)         
< >   Coda file system support (advanced network fs)             
< >   Andrew File System support (AFS) (EXPERIMENTAL)  

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then recompile kernel
Now the nfs-kernel-server start successfully.

2. error:
exportfs: Warning: /nfs does not support NFS export.