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Linus Torvalds1da177e2005-04-16 15:20:36 -07001
2Making Filesystems Exportable
3=============================
4
Christoph Hellwige38f9812007-10-21 16:42:19 -07005Overview
6--------
7
8All filesystem operations require a dentry (or two) as a starting
Linus Torvalds1da177e2005-04-16 15:20:36 -07009point. Local applications have a reference-counted hold on suitable
Christoph Hellwige38f9812007-10-21 16:42:19 -070010dentries via open file descriptors or cwd/root. However remote
Linus Torvalds1da177e2005-04-16 15:20:36 -070011applications 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
Christoph Hellwige38f9812007-10-21 16:42:19 -070019specify how to generate an opaque (outside of the filesystem) byte
Linus Torvalds1da177e2005-04-16 15:20:36 -070020string 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
Christoph Hellwige38f9812007-10-21 16:42:19 -070026and dentries will be termed "exportable".
Linus Torvalds1da177e2005-04-16 15:20:36 -070027
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:
J. Bruce Fields96353892014-02-18 12:31:31 -050069 d_obtain_alias(inode) will return a dentry for the given inode.
Linus Torvalds1da177e2005-04-16 15:20:36 -070070 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.
Al Viro41d28bc2014-10-12 22:24:21 -040075 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.
J. Bruce Fields96353892014-02-18 12:31:31 -050080
Linus Torvalds1da177e2005-04-16 15:20:36 -070081
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.
Phillip Lougher5b9f4562011-07-26 03:40:45 +010090
Masanari Iida9ed354b2013-08-20 20:33:17 +090091 If inode is NULL, d_splice_alias(inode, dentry) is equivalent to
Phillip Lougher5b9f4562011-07-26 03:40:45 +010092
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:
Christoph Hellwige38f9812007-10-21 16:42:19 -070098
99 return d_splice_alias(inode, dentry);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700100 }
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"
Christoph Hellwige38f9812007-10-21 16:42:19 -0700107struct which has the following members:
Linus Torvalds1da177e2005-04-16 15:20:36 -0700108
Christoph Hellwige38f9812007-10-21 16:42:19 -0700109 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.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700115
Christoph Hellwige38f9812007-10-21 16:42:19 -0700116 fh_to_dentry (mandatory)
117 Given a filehandle fragment, this should find the implied object and
J. Bruce Fields96353892014-02-18 12:31:31 -0500118 create a dentry for it (possibly with d_obtain_alias).
Linus Torvalds1da177e2005-04-16 15:20:36 -0700119
Christoph Hellwige38f9812007-10-21 16:42:19 -0700120 fh_to_parent (optional but strongly recommended)
121 Given a filehandle fragment, this should find the parent of the
J. Bruce Fields96353892014-02-18 12:31:31 -0500122 implied object and create a dentry for it (possibly with
123 d_obtain_alias). May fail if the filehandle fragment is too small.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700124
Christoph Hellwige38f9812007-10-21 16:42:19 -0700125 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.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700132
Christoph Hellwige38f9812007-10-21 16:42:19 -0700133 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.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700140
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.