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Based on kernel version 4.13.3. Page generated on 2017-09-23 13:55 EST.

2	Making Filesystems Exportable
3	=============================
5	Overview
6	--------
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".
30	Dcache Issues
31	-------------
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
38	its parent).
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
59	   prefix. 
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
65	   d_hash list_head.
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
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.
82	Filesystem Issues
83	-----------------
85	For a filesystem to be exportable it must:
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.
91	      If inode is NULL, d_splice_alias(inode, dentry) is equivalent to
93			d_add(dentry, inode), NULL
95	      Similarly, d_splice_alias(ERR_PTR(err), dentry) = ERR_PTR(err)
97	      Typically the ->lookup routine will simply end with a:
99			return d_splice_alias(inode, dentry);
100		}
104	  A file system implementation declares that instances of the filesystem
105	are exportable by setting the s_export_op field in the struct
106	super_block.  This field must point to a "struct export_operations"
107	struct which has the following members:
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.
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).
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.
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.
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.
142	A filehandle fragment consists of an array of 1 or more 4byte words,
143	together with a one byte "type".
144	The decode_fh routine should not depend on the stated size that is
145	passed to it.  This size may be larger than the original filehandle
146	generated by encode_fh, in which case it will have been padded with
147	nuls.  Rather, the encode_fh routine should choose a "type" which
148	indicates the decode_fh how much of the filehandle is valid, and how
149	it should be interpreted.
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