# Copyright (C) 2005-2014 Junjiro R. Okajima
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Export Aufs via NFS
Here is an approach.
- like xino/xib, add a new file 'xigen' which stores aufs inode
- 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
+ file handle for a branch (by exportfs_encode_fh())
+ 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.