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Documentation / filesystems / nfs / Exporting


Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 EST.

1	
2	Making Filesystems Exportable
3	=============================
4	
5	Overview
6	--------
7	
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'.
17	
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.
24	
25	A filesystem which supports the mapping between filehandle fragments
26	and dentries will be termed "exportable".
27	
28	
29	
30	Dcache Issues
31	-------------
32	
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).
39	
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.
44	
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.
52	
53	To implement these features, the dcache has:
54	
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.  If the refcount on a dentry with this flag set
60	   becomes zero, the dentry is immediately discarded, rather than being
61	   kept in the dcache.  If a dentry that is not already in the dcache
62	   is repeatedly accessed by filehandle (as NFSD might do), an new dentry
63	   will be a allocated for each access, and discarded at the end of
64	   the access.
65	
66	   Note that such a dentry can acquire children, name, ancestors, etc.
67	   without losing DCACHE_DISCONNECTED - that flag is only cleared when
68	   subtree is successfully reconnected to root.  Until then dentries
69	   in such subtree are retained only as long as there are references;
70	   refcount reaching zero means immediate eviction, same as for unhashed
71	   dentries.  That guarantees that we won't need to hunt them down upon
72	   umount.
73	
74	b/ A primitive for creation of secondary roots - d_obtain_root(inode).
75	   Those do _not_ bear DCACHE_DISCONNECTED.  They are placed on the
76	   per-superblock list (->s_roots), so they can be located at umount
77	   time for eviction purposes.
78	
79	c/ Helper routines to allocate anonymous dentries, and to help attach
80	   loose directory dentries at lookup time. They are:
81	    d_obtain_alias(inode) will return a dentry for the given inode.
82	      If the inode already has a dentry, one of those is returned.
83	      If it doesn't, a new anonymous (IS_ROOT and
84	        DCACHE_DISCONNECTED) dentry is allocated and attached.
85	      In the case of a directory, care is taken that only one dentry
86	      can ever be attached.
87	    d_splice_alias(inode, dentry) will introduce a new dentry into the tree;
88	      either the passed-in dentry or a preexisting alias for the given inode
89	      (such as an anonymous one created by d_obtain_alias), if appropriate.
90	      It returns NULL when the passed-in dentry is used, following the calling
91	      convention of ->lookup.
92	 
93	Filesystem Issues
94	-----------------
95	
96	For a filesystem to be exportable it must:
97	 
98	   1/ provide the filehandle fragment routines described below.
99	   2/ make sure that d_splice_alias is used rather than d_add
100	      when ->lookup finds an inode for a given parent and name.
101	
102	      If inode is NULL, d_splice_alias(inode, dentry) is equivalent to
103	
104			d_add(dentry, inode), NULL
105	
106	      Similarly, d_splice_alias(ERR_PTR(err), dentry) = ERR_PTR(err)
107	
108	      Typically the ->lookup routine will simply end with a:
109	
110			return d_splice_alias(inode, dentry);
111		}
112	
113	
114	
115	  A file system implementation declares that instances of the filesystem
116	are exportable by setting the s_export_op field in the struct
117	super_block.  This field must point to a "struct export_operations"
118	struct which has the following members:
119	
120	 encode_fh  (optional)
121	    Takes a dentry and creates a filehandle fragment which can later be used
122	    to find or create a dentry for the same object.  The default
123	    implementation creates a filehandle fragment that encodes a 32bit inode
124	    and generation number for the inode encoded, and if necessary the
125	    same information for the parent.
126	
127	  fh_to_dentry (mandatory)
128	    Given a filehandle fragment, this should find the implied object and
129	    create a dentry for it (possibly with d_obtain_alias).
130	
131	  fh_to_parent (optional but strongly recommended)
132	    Given a filehandle fragment, this should find the parent of the
133	    implied object and create a dentry for it (possibly with
134	    d_obtain_alias).  May fail if the filehandle fragment is too small.
135	
136	  get_parent (optional but strongly recommended)
137	    When given a dentry for a directory, this should return  a dentry for
138	    the parent.  Quite possibly the parent dentry will have been allocated
139	    by d_alloc_anon.  The default get_parent function just returns an error
140	    so any filehandle lookup that requires finding a parent will fail.
141	    ->lookup("..") is *not* used as a default as it can leave ".." entries
142	    in the dcache which are too messy to work with.
143	
144	  get_name (optional)
145	    When given a parent dentry and a child dentry, this should find a name
146	    in the directory identified by the parent dentry, which leads to the
147	    object identified by the child dentry.  If no get_name function is
148	    supplied, a default implementation is provided which uses vfs_readdir
149	    to find potential names, and matches inode numbers to find the correct
150	    match.
151	
152	
153	A filehandle fragment consists of an array of 1 or more 4byte words,
154	together with a one byte "type".
155	The decode_fh routine should not depend on the stated size that is
156	passed to it.  This size may be larger than the original filehandle
157	generated by encode_fh, in which case it will have been padded with
158	nuls.  Rather, the encode_fh routine should choose a "type" which
159	indicates the decode_fh how much of the filehandle is valid, and how
160	it should be interpreted.
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