2 Making Filesystems Exportable
3 =============================
8 All filesystem operations require a dentry (or two) as a starting
9 point. Local applications have a reference-counted hold on suitable
10 dentries via open file descriptors or cwd/root. However remote
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
14 form of reference needs to be stable across renames, truncates, and
15 server-reboot (among other things, though these tend to be the most
16 problematic), there is no simple answer like 'filename'.
18 The mechanism discussed here allows each filesystem implementation to
19 specify how to generate an opaque (outside of the filesystem) byte
20 string for any dentry, and how to find an appropriate dentry for any
21 given opaque byte string.
22 This byte string will be called a "filehandle fragment" as it
23 corresponds to part of an NFS filehandle.
25 A filesystem which supports the mapping between filehandle fragments
26 and dentries will be termed "exportable".
33 The dcache normally contains a proper prefix of any given filesystem
34 tree. This means that if any filesystem object is in the dcache, then
35 all of the ancestors of that filesystem object are also in the dcache.
36 As normal access is by filename this prefix is created naturally and
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
42 for the object. This leads to two related but distinct features of
43 the dcache that are not needed for normal filesystem access.
45 1/ The dcache must sometimes contain objects that are not part of the
46 proper prefix. i.e that are not connected to the root.
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
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.
53 To implement these features, the dcache has:
55 a/ 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
61 b/ 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
67 c/ 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
75 d_splice_alias(inode, dentry) or d_materialise_unique(dentry, inode)
76 will introduce a new dentry into the tree; either the passed-in
77 dentry or a preexisting alias for the given inode (such as an
78 anonymous one created by d_obtain_alias), if appropriate. The two
79 functions differ in their handling of directories with preexisting
81 d_splice_alias will use any existing IS_ROOT dentry, but it will
82 return -EIO rather than try to move a dentry with a different
83 parent. This is appropriate for local filesystems, which
84 should never see such an alias unless the filesystem is
85 corrupted somehow (for example, if two on-disk directory
86 entries refer to the same directory.)
87 d_materialise_unique will attempt to move any dentry. This is
88 appropriate for distributed filesystems, where finding a
89 directory other than where we last cached it may be a normal
90 consequence of concurrent operations on other hosts.
91 Both functions return NULL when the passed-in dentry is used,
92 following the calling convention of ->lookup.
98 For a filesystem to be exportable it must:
100 1/ provide the filehandle fragment routines described below.
101 2/ make sure that d_splice_alias is used rather than d_add
102 when ->lookup finds an inode for a given parent and name.
104 If inode is NULL, d_splice_alias(inode, dentry) is equivalent to
106 d_add(dentry, inode), NULL
108 Similarly, d_splice_alias(ERR_PTR(err), dentry) = ERR_PTR(err)
110 Typically the ->lookup routine will simply end with a:
112 return d_splice_alias(inode, dentry);
117 A file system implementation declares that instances of the filesystem
118 are exportable by setting the s_export_op field in the struct
119 super_block. This field must point to a "struct export_operations"
120 struct which has the following members:
123 Takes a dentry and creates a filehandle fragment which can later be used
124 to find or create a dentry for the same object. The default
125 implementation creates a filehandle fragment that encodes a 32bit inode
126 and generation number for the inode encoded, and if necessary the
127 same information for the parent.
129 fh_to_dentry (mandatory)
130 Given a filehandle fragment, this should find the implied object and
131 create a dentry for it (possibly with d_obtain_alias).
133 fh_to_parent (optional but strongly recommended)
134 Given a filehandle fragment, this should find the parent of the
135 implied object and create a dentry for it (possibly with
136 d_obtain_alias). May fail if the filehandle fragment is too small.
138 get_parent (optional but strongly recommended)
139 When given a dentry for a directory, this should return a dentry for
140 the parent. Quite possibly the parent dentry will have been allocated
141 by d_alloc_anon. The default get_parent function just returns an error
142 so any filehandle lookup that requires finding a parent will fail.
143 ->lookup("..") is *not* used as a default as it can leave ".." entries
144 in the dcache which are too messy to work with.
147 When given a parent dentry and a child dentry, this should find a name
148 in the directory identified by the parent dentry, which leads to the
149 object identified by the child dentry. If no get_name function is
150 supplied, a default implementation is provided which uses vfs_readdir
151 to find potential names, and matches inode numbers to find the correct
155 A filehandle fragment consists of an array of 1 or more 4byte words,
156 together with a one byte "type".
157 The decode_fh routine should not depend on the stated size that is
158 passed to it. This size may be larger than the original filehandle
159 generated by encode_fh, in which case it will have been padded with
160 nuls. Rather, the encode_fh routine should choose a "type" which
161 indicates the decode_fh how much of the filehandle is valid, and how
162 it should be interpreted.