I tried to track the difference between overlay-NFSv3 and ext4-NFSv3
of encode_post_op_attr.
mount configuration:
none /share overlay
rw,relatime,lowerdir=/base/lower,upperdir=/base/upper,workdir=/base/work,index=on,nfs_export=on
0 0
localhost:/share /mnt/n nfs
rw,relatime,vers=3,rsize=1048576,wsize=1048576,namlen=255,hard,proto=tcp,timeo=600,retrans=2,sec=sys,mountaddr=127.0.0.1,mountvers=3,mountport=40931,mountproto=udp,local_lock=none,addr=127.0.0.1
0 0
/dev/loop0 /share2 ext4 rw,relatime,data=ordered 0 0
localhost:/share2 /mnt/n2 nfs
rw,relatime,vers=3,rsize=1048576,wsize=1048576,namlen=255,hard,proto=tcp,timeo=600,retrans=2,sec=sys,mountaddr=127.0.0.1,mountvers=3,mountport=40931,mountproto=udp,local_lock=none,addr=127.0.0.1
0 0

file tree:
/mnt/n
|-- dirA
|   `-- bar
|-- dirL
|   `-- ro-file
`-- foo

/mnt/n2
`-- lost+found

Attached log are dmesg start from "readdir /mnt/n (n2)" with nfs and
nfsd log are enabled all by sunrpc.nfs(d)_debug. I also add a
dump_stack() in the beginning of encode_post_op_attr.

It seems overlay and ext4 have different call flow after "nfsd:
READDIR+", there is no failure in overlay's encode_post_op_attr, nfsd
does fill the attrs, same as ext4 but it has additional readdir call
to child node of "/". I'm not sure if this is normal to overlay and is
the cause of DT_UNKNOWN.

In additional, overlay returns DT_UNKNOWN to either lower dir or merged dir.

/readdir/a.out /mnt/n
readdir: dirL ino(e007c) type(0)
readdir: . ino(e000c) type(0)
readdir: .. ino(e000c) type(0)
readdir: foo ino(e0005) type(0)
readdir: dirA ino(e0006) type(0)

/readdir/a.out /mnt/n2
readdir: lost+found ino(b) type(4)
readdir: .. ino(2) type(0)
readdir: . ino(2) type(4)

thanks,
Eddie