1
2Making Filesystems Exportable
3=============================
4
5Overview
6--------
7
8All filesystem operations require a dentry (or two) as a starting
9point.  Local applications have a reference-counted hold on suitable
10dentries via open file descriptors or cwd/root.  However remote
11applications that access a filesystem via a remote filesystem protocol
12such as NFS may not be able to hold such a reference, and so need a
13different way to refer to a particular dentry.  As the alternative
14form of reference needs to be stable across renames, truncates, and
15server-reboot (among other things, though these tend to be the most
16problematic), there is no simple answer like 'filename'.
17
18The mechanism discussed here allows each filesystem implementation to
19specify how to generate an opaque (outside of the filesystem) byte
20string for any dentry, and how to find an appropriate dentry for any
21given opaque byte string.
22This byte string will be called a "filehandle fragment" as it
23corresponds to part of an NFS filehandle.
24
25A filesystem which supports the mapping between filehandle fragments
26and dentries will be termed "exportable".
27
28
29
30Dcache Issues
31-------------
32
33The dcache normally contains a proper prefix of any given filesystem
34tree.  This means that if any filesystem object is in the dcache, then
35all of the ancestors of that filesystem object are also in the dcache.
36As normal access is by filename this prefix is created naturally and
37maintained easily (by each object maintaining a reference count on
38its parent).
39
40However when objects are included into the dcache by interpreting a
41filehandle fragment, there is no automatic creation of a path prefix
42for the object.  This leads to two related but distinct features of
43the dcache that are not needed for normal filesystem access.
44
451/ The dcache must sometimes contain objects that are not part of the
46   proper prefix. i.e that are not connected to the root.
472/ 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
49   that dentry into place (based on the parent and name in the
50   ->lookup).   This is particularly needed for directories as
51   it is a dcache invariant that directories only have one dentry.
52
53To implement these features, the dcache has:
54
55a/ A dentry flag DCACHE_DISCONNECTED which is set on
56   any dentry that might not be part of the proper prefix.
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
59   prefix. 
60
61b/ A per-superblock list "s_anon" of dentries which are the roots of
62   subtrees that are not in the proper prefix.  These dentries, as
63   well as the proper prefix, need to be released at unmount time.  As
64   these dentries will not be hashed, they are linked together on the
65   d_hash list_head.
66
67c/ Helper routines to allocate anonymous dentries, and to help attach
68   loose directory dentries at lookup time. They are:
69    d_obtain_alias(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
72        DCACHE_DISCONNECTED) dentry is allocated and attached.
73      In the case of a directory, care is taken that only one dentry
74      can ever be attached.
75    d_splice_alias(inode, dentry) will introduce a new dentry into the tree;
76      either the passed-in dentry or a preexisting alias for the given inode
77      (such as an anonymous one created by d_obtain_alias), if appropriate.
78      It returns NULL when the passed-in dentry is used, following the calling
79      convention of ->lookup.
80
81 
82Filesystem Issues
83-----------------
84
85For a filesystem to be exportable it must:
86 
87   1/ provide the filehandle fragment routines described below.
88   2/ make sure that d_splice_alias is used rather than d_add
89      when ->lookup finds an inode for a given parent and name.
90
91      If inode is NULL, d_splice_alias(inode, dentry) is equivalent to
92
93		d_add(dentry, inode), NULL
94
95      Similarly, d_splice_alias(ERR_PTR(err), dentry) = ERR_PTR(err)
96
97      Typically the ->lookup routine will simply end with a:
98
99		return d_splice_alias(inode, dentry);
100	}
101
102
103
104  A file system implementation declares that instances of the filesystem
105are exportable by setting the s_export_op field in the struct
106super_block.  This field must point to a "struct export_operations"
107struct which has the following members:
108
109 encode_fh  (optional)
110    Takes a dentry and creates a filehandle fragment which can later be used
111    to find or create a dentry for the same object.  The default
112    implementation creates a filehandle fragment that encodes a 32bit inode
113    and generation number for the inode encoded, and if necessary the
114    same information for the parent.
115
116  fh_to_dentry (mandatory)
117    Given a filehandle fragment, this should find the implied object and
118    create a dentry for it (possibly with d_obtain_alias).
119
120  fh_to_parent (optional but strongly recommended)
121    Given a filehandle fragment, this should find the parent of the
122    implied object and create a dentry for it (possibly with
123    d_obtain_alias).  May fail if the filehandle fragment is too small.
124
125  get_parent (optional but strongly recommended)
126    When given a dentry for a directory, this should return  a dentry for
127    the parent.  Quite possibly the parent dentry will have been allocated
128    by d_alloc_anon.  The default get_parent function just returns an error
129    so any filehandle lookup that requires finding a parent will fail.
130    ->lookup("..") is *not* used as a default as it can leave ".." entries
131    in the dcache which are too messy to work with.
132
133  get_name (optional)
134    When given a parent dentry and a child dentry, this should find a name
135    in the directory identified by the parent dentry, which leads to the
136    object identified by the child dentry.  If no get_name function is
137    supplied, a default implementation is provided which uses vfs_readdir
138    to find potential names, and matches inode numbers to find the correct
139    match.
140
141
142A filehandle fragment consists of an array of 1 or more 4byte words,
143together with a one byte "type".
144The decode_fh routine should not depend on the stated size that is
145passed to it.  This size may be larger than the original filehandle
146generated by encode_fh, in which case it will have been padded with
147nuls.  Rather, the encode_fh routine should choose a "type" which
148indicates the decode_fh how much of the filehandle is valid, and how
149it should be interpreted.
150