2 Making Filesystems Exportable
3 =============================
5 Most filesystem operations require a dentry (or two) as a starting
6 point. Local applications have a reference-counted hold on suitable
7 dentrys via open file descriptors or cwd/root. However remote
8 applications that access a filesystem via a remote filesystem protocol
9 such as NFS may not be able to hold such a reference, and so need a
10 different way to refer to a particular dentry. As the alternative
11 form of reference needs to be stable across renames, truncates, and
12 server-reboot (among other things, though these tend to be the most
13 problematic), there is no simple answer like 'filename'.
15 The mechanism discussed here allows each filesystem implementation to
16 specify how to generate an opaque (out side of the filesystem) byte
17 string for any dentry, and how to find an appropriate dentry for any
18 given opaque byte string.
19 This byte string will be called a "filehandle fragment" as it
20 corresponds to part of an NFS filehandle.
22 A filesystem which supports the mapping between filehandle fragments
23 and dentrys will be termed "exportable".
30 The dcache normally contains a proper prefix of any given filesystem
31 tree. This means that if any filesystem object is in the dcache, then
32 all of the ancestors of that filesystem object are also in the dcache.
33 As normal access is by filename this prefix is created naturally and
34 maintained easily (by each object maintaining a reference count on
37 However when objects are included into the dcache by interpreting a
38 filehandle fragment, there is no automatic creation of a path prefix
39 for the object. This leads to two related but distinct features of
40 the dcache that are not needed for normal filesystem access.
42 1/ The dcache must sometimes contain objects that are not part of the
43 proper prefix. i.e that are not connected to the root.
44 2/ The dcache must be prepared for a newly found (via ->lookup) directory
45 to already have a (non-connected) dentry, and must be able to move
46 that dentry into place (based on the parent and name in the
47 ->lookup). This is particularly needed for directories as
48 it is a dcache invariant that directories only have one dentry.
50 To implement these features, the dcache has:
52 a/ A dentry flag DCACHE_DISCONNECTED which is set on
53 any dentry that might not be part of the proper prefix.
54 This is set when anonymous dentries are created, and cleared when a
55 dentry is noticed to be a child of a dentry which is in the proper
58 b/ A per-superblock list "s_anon" of dentries which are the roots of
59 subtrees that are not in the proper prefix. These dentries, as
60 well as the proper prefix, need to be released at unmount time. As
61 these dentries will not be hashed, they are linked together on the
64 c/ Helper routines to allocate anonymous dentries, and to help attach
65 loose directory dentries at lookup time. They are:
66 d_alloc_anon(inode) will return a dentry for the given inode.
67 If the inode already has a dentry, one of those is returned.
68 If it doesn't, a new anonymous (IS_ROOT and
69 DCACHE_DISCONNECTED) dentry is allocated and attached.
70 In the case of a directory, care is taken that only one dentry
72 d_splice_alias(inode, dentry) will make sure that there is a
73 dentry with the same name and parent as the given dentry, and
74 which refers to the given inode.
75 If the inode is a directory and already has a dentry, then that
76 dentry is d_moved over the given dentry.
77 If the passed dentry gets attached, care is taken that this is
78 mutually exclusive to a d_alloc_anon operation.
79 If the passed dentry is used, NULL is returned, else the used
80 dentry is returned. This corresponds to the calling pattern of
87 For a filesystem to be exportable it must:
89 1/ provide the filehandle fragment routines described below.
90 2/ make sure that d_splice_alias is used rather than d_add
91 when ->lookup finds an inode for a given parent and name.
92 Typically the ->lookup routine will end:
94 return d_splice(inode, dentry);
101 A file system implementation declares that instances of the filesystem
102 are exportable by setting the s_export_op field in the struct
103 super_block. This field must point to a "struct export_operations"
104 struct which could potentially be full of NULLs, though normally at
105 least get_parent will be set.
107 The primary operations are decode_fh and encode_fh.
108 decode_fh takes a filehandle fragment and tries to find or create a
109 dentry for the object referred to by the filehandle.
110 encode_fh takes a dentry and creates a filehandle fragment which can
111 later be used to find/create a dentry for the same object.
113 decode_fh will probably make use of "find_exported_dentry".
114 This function lives in the "exportfs" module which a filesystem does
115 not need unless it is being exported. So rather that calling
116 find_exported_dentry directly, each filesystem should call it through
117 the find_exported_dentry pointer in it's export_operations table.
118 This field is set correctly by the exporting agent (e.g. nfsd) when a
119 filesystem is exported, and before any export operations are called.
121 find_exported_dentry needs three support functions from the
123 get_name. When given a parent dentry and a child dentry, this
124 should find a name in the directory identified by the parent
125 dentry, which leads to the object identified by the child dentry.
126 If no get_name function is supplied, a default implementation is
127 provided which uses vfs_readdir to find potential names, and
128 matches inode numbers to find the correct match.
130 get_parent. When given a dentry for a directory, this should return
131 a dentry for the parent. Quite possibly the parent dentry will
132 have been allocated by d_alloc_anon.
133 The default get_parent function just returns an error so any
134 filehandle lookup that requires finding a parent will fail.
135 ->lookup("..") is *not* used as a default as it can leave ".."
136 entries in the dcache which are too messy to work with.
138 get_dentry. When given an opaque datum, this should find the
139 implied object and create a dentry for it (possibly with
141 The opaque datum is whatever is passed down by the decode_fh
142 function, and is often simply a fragment of the filehandle
144 decode_fh passes two datums through find_exported_dentry. One that
145 should be used to identify the target object, and one that can be
146 used to identify the object's parent, should that be necessary.
147 The default get_dentry function assumes that the datum contains an
148 inode number and a generation number, and it attempts to get the
149 inode using "iget" and check it's validity by matching the
150 generation number. A filesystem should only depend on the default
151 if iget can safely be used this way.
153 If decode_fh and/or encode_fh are left as NULL, then default
154 implementations are used. These defaults are suitable for ext2 and
155 extremely similar filesystems (like ext3).
157 The default encode_fh creates a filehandle fragment from the inode
158 number and generation number of the target together with the inode
159 number and generation number of the parent (if the parent is
162 The default decode_fh extract the target and parent datums from the
163 filehandle assuming the format used by the default encode_fh and
164 passed them to find_exported_dentry.
167 A filehandle fragment consists of an array of 1 or more 4byte words,
168 together with a one byte "type".
169 The decode_fh routine should not depend on the stated size that is
170 passed to it. This size may be larger than the original filehandle
171 generated by encode_fh, in which case it will have been padded with
172 nuls. Rather, the encode_fh routine should choose a "type" which
173 indicates the decode_fh how much of the filehandle is valid, and how
174 it should be interpreted.
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