Documentation/filesystems/nfs/Exporting

   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.
  60
  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.
  66
  67 c/ Helper routines to allocate anonymous dentries, and to help attach
  68    loose directory dentries at lookup time. They are:
  69     d_alloc_anon(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 make sure that there is a
  76       dentry with the same name and parent as the given dentry, and
  77       which refers to the given inode.
  78       If the inode is a directory and already has a dentry, then that
  79       dentry is d_moved over the given dentry.
  80       If the passed dentry gets attached, care is taken that this is
  81       mutually exclusive to a d_alloc_anon operation.
  82       If the passed dentry is used, NULL is returned, else the used
  83       dentry is returned.  This corresponds to the calling pattern of
  84       ->lookup.
  85
  86
  87 Filesystem Issues
  88 -----------------
  89
  90 For a filesystem to be exportable it must:
  91
  92    1/ provide the filehandle fragment routines described below.
  93    2/ make sure that d_splice_alias is used rather than d_add
  94       when ->lookup finds an inode for a given parent and name.
  95
  96       If inode is NULL, d_splice_alias(inode, dentry) is equivalent to
  97
  98                 d_add(dentry, inode), NULL
  99
 100       Similarly, d_splice_alias(ERR_PTR(err), dentry) = ERR_PTR(err)
 101
 102       Typically the ->lookup routine will simply end with a:
 103
 104                 return d_splice_alias(inode, dentry);
 105         }
 106
 107
 108
 109   A file system implementation declares that instances of the filesystem
 110 are exportable by setting the s_export_op field in the struct
 111 super_block.  This field must point to a "struct export_operations"
 112 struct which has the following members:
 113
 114  encode_fh  (optional)
 115     Takes a dentry and creates a filehandle fragment which can later be used
 116     to find or create a dentry for the same object.  The default
 117     implementation creates a filehandle fragment that encodes a 32bit inode
 118     and generation number for the inode encoded, and if necessary the
 119     same information for the parent.
 120
 121   fh_to_dentry (mandatory)
 122     Given a filehandle fragment, this should find the implied object and
 123     create a dentry for it (possibly with d_alloc_anon).
 124
 125   fh_to_parent (optional but strongly recommended)
 126     Given a filehandle fragment, this should find the parent of the
 127     implied object and create a dentry for it (possibly with d_alloc_anon).
 128     May fail if the filehandle fragment is too small.
 129
 130   get_parent (optional but strongly recommended)
 131     When given a dentry for a directory, this should return  a dentry for
 132     the parent.  Quite possibly the parent dentry will have been allocated
 133     by d_alloc_anon.  The default get_parent function just returns an error
 134     so any filehandle lookup that requires finding a parent will fail.
 135     ->lookup("..") is *not* used as a default as it can leave ".." entries
 136     in the dcache which are too messy to work with.
 137
 138   get_name (optional)
 139     When given a parent dentry and a child dentry, this should find a name
 140     in the directory identified by the parent dentry, which leads to the
 141     object identified by the child dentry.  If no get_name function is
 142     supplied, a default implementation is provided which uses vfs_readdir
 143     to find potential names, and matches inode numbers to find the correct
 144     match.
 145
 146
 147 A filehandle fragment consists of an array of 1 or more 4byte words,
 148 together with a one byte "type".
 149 The decode_fh routine should not depend on the stated size that is
 150 passed to it.  This size may be larger than the original filehandle
 151 generated by encode_fh, in which case it will have been padded with
 152 nuls.  Rather, the encode_fh routine should choose a "type" which
 153 indicates the decode_fh how much of the filehandle is valid, and how
 154 it should be interpreted.