Fluentd tg-agent can't access file in NFS path

Configuration file as

<source>
  @type tail
  format csv
  keys ARRIVAL_DATE
  path /nfs/file.txt
  tag product
  pos_file /nfs/file.pos
</source>

nfs path and permission:

ls -la /nfs/file.txt
-rw-rw----. 1 909 900 3904  8 31 13:20 /nfs/file.txt

ls -la /nfs/file.pos
-rw-r--r--. 1 909 900 50  6 23  2015 /nfs/file.pos

In the fluentd log:

unexpected error error_class=Errno::EACCES error="Permission denied @ rb_sysopen - /nfs/file.pos"

What’s the right permission for file in the NFS mounted path?