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8 All filesystem operations require a dentry (or two) as a starting
9 point.  Local applications have a reference-counted hold on suitable
11 applications that access a filesystem via a remote filesystem protocol
12 such as NFS may not be able to hold such a reference, and so need a
13 different way to refer to a particular dentry.  As the alternative
22 This byte string will be called a "filehandle fragment" as it
25 A filesystem which supports the mapping between filehandle fragments
33 The dcache normally contains a proper prefix of any given filesystem
37 maintained easily (by each object maintaining a reference count on
40 However when objects are included into the dcache by interpreting a
41 filehandle fragment, there is no automatic creation of a path prefix
47 2/ The dcache must be prepared for a newly found (via ->lookup) directory
48    to already have a (non-connected) dentry, and must be able to move
51    it is a dcache invariant that directories only have one dentry.
55 a/ A dentry flag DCACHE_DISCONNECTED which is set on
57    This is set when anonymous dentries are created, and cleared when a
58    dentry is noticed to be a child of a dentry which is in the proper
61 b/ A per-superblock list "s_anon" of dentries which are the roots of
69     d_alloc_anon(inode) will return a dentry for the given inode.
70       If the inode already has a dentry, one of those is returned.
71       If it doesn't, a new anonymous (IS_ROOT and
73       In the case of a directory, care is taken that only one dentry
75     d_splice_alias(inode, dentry) will make sure that there is a
78       If the inode is a directory and already has a dentry, then that
81       mutually exclusive to a d_alloc_anon operation.
90 For a filesystem to be exportable it must:
94       when ->lookup finds an inode for a given parent and name.
102       Typically the ->lookup routine will simply end with a:
109   A file system implementation declares that instances of the filesystem
111 super_block.  This field must point to a "struct export_operations"
115     Takes a dentry and creates a filehandle fragment which can later be used
116     to find or create a dentry for the same object.  The default
117     implementation creates a filehandle fragment that encodes a 32bit inode
122     Given a filehandle fragment, this should find the implied object and
123     create a dentry for it (possibly with d_alloc_anon).
126     Given a filehandle fragment, this should find the parent of the
127     implied object and create a dentry for it (possibly with d_alloc_anon).
131     When given a dentry for a directory, this should return  a dentry for
134     so any filehandle lookup that requires finding a parent will fail.
135     ->lookup("..") is *not* used as a default as it can leave ".." entries
139     When given a parent dentry and a child dentry, this should find a name
142     supplied, a default implementation is provided which uses vfs_readdir
147 A filehandle fragment consists of an array of 1 or more 4byte words,
148 together with a one byte "type".
152 nuls.  Rather, the encode_fh routine should choose a "type" which