# Copyright (C) 2005-2014 Junjiro R. Okajima

Export Aufs via NFS
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Here is an approach.
- like xino/xib, add a new file 'xigen' which stores aufs inode
  generation.
- iget_locked(): initialize aufs inode generation for a new inode, and
  store it in xigen file.
- destroy_inode(): increment aufs inode generation and store it in xigen
  file. it is necessary even if it is not unlinked, because any data of
  inode may be changed by UDBA.
- encode_fh(): for a root dir, simply return FILEID_ROOT. otherwise
  build file handle by
  + branch id (4 bytes)
  + superblock generation (4 bytes)
  + inode number (4 or 8 bytes)
  + parent dir inode number (4 or 8 bytes)
  + inode generation (4 bytes))
  + return value of exportfs_encode_fh() for the parent on a branch (4
    bytes)
  + file handle for a branch (by exportfs_encode_fh())
- fh_to_dentry():
  + find the index of a branch from its id in handle, and check it is
    still exist in aufs.
  + 1st level: get the inode number from handle and search it in cache.
  + 2nd level: if not found, get the parent inode number from handle and
    search it in cache. and then open the parent dir, find the matching
    inode number by vfs_readdir() and get its name, and call
    lookup_one_len() for the target dentry.
  + 3rd level: if the parent dir is not cached, call
    exportfs_decode_fh() for a branch and get the parent on a branch,
    build a pathname of it, convert it a pathname in aufs, call
    path_lookup(). now aufs gets a parent dir dentry, then handle it as
    the 2nd level.
  + to open the dir, aufs needs struct vfsmount. aufs keeps vfsmount
    for every branch, but not itself. to get this, (currently) aufs
    searches in current->nsproxy->mnt_ns list. it may not be a good
    idea, but I didn't get other approach.
  + test the generation of the gotten inode.
- every inode operation: they may get EBUSY due to UDBA. in this case,
  convert it into ESTALE for NFSD.
- readdir(): call lockdep_on/off() because filldir in NFSD calls
  lookup_one_len(), vfs_getattr(), encode_fh() and others.