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 Making Filesystems Exportable
 =============================
 
 Overview
 --------
 
 All filesystem operations require a dentry (or two) as a starting
 point.  Local applications have a reference-counted hold on suitable
 dentries via open file descriptors or cwd/root.  However remote
 applications that access a filesystem via a remote filesystem protocol
 such as NFS may not be able to hold such a reference, and so need a
 different way to refer to a particular dentry.  As the alternative
 form of reference needs to be stable across renames, truncates, and
 server-reboot (among other things, though these tend to be the most
 problematic), there is no simple answer like 'filename'.
 
 The mechanism discussed here allows each filesystem implementation to
 specify how to generate an opaque (outside of the filesystem) byte
 string for any dentry, and how to find an appropriate dentry for any
 given opaque byte string.
 This byte string will be called a "filehandle fragment" as it
 corresponds to part of an NFS filehandle.
 
 A filesystem which supports the mapping between filehandle fragments
 and dentries will be termed "exportable".
 
 
 
 Dcache Issues
 -------------
 
 The dcache normally contains a proper prefix of any given filesystem
 tree.  This means that if any filesystem object is in the dcache, then
 all of the ancestors of that filesystem object are also in the dcache.
 As normal access is by filename this prefix is created naturally and
 maintained easily (by each object maintaining a reference count on
 its parent).
 
 However when objects are included into the dcache by interpreting a
 filehandle fragment, there is no automatic creation of a path prefix
 for the object.  This leads to two related but distinct features of
 the dcache that are not needed for normal filesystem access.
 
 1. The dcache must sometimes contain objects that are not part of the
    proper prefix. i.e that are not connected to the root.
 2. The dcache must be prepared for a newly found (via ->lookup) directory
    to already have a (non-connected) dentry, and must be able to move
    that dentry into place (based on the parent and name in the
    ->lookup).   This is particularly needed for directories as
    it is a dcache invariant that directories only have one dentry.
 
 To implement these features, the dcache has:
 
 a. A dentry flag DCACHE_DISCONNECTED which is set on
    any dentry that might not be part of the proper prefix.
    This is set when anonymous dentries are created, and cleared when a
    dentry is noticed to be a child of a dentry which is in the proper
    prefix.  If the refcount on a dentry with this flag set
    becomes zero, the dentry is immediately discarded, rather than being
    kept in the dcache.  If a dentry that is not already in the dcache
    is repeatedly accessed by filehandle (as NFSD might do), an new dentry
    will be a allocated for each access, and discarded at the end of
    the access.
 
    Note that such a dentry can acquire children, name, ancestors, etc.
    without losing DCACHE_DISCONNECTED - that flag is only cleared when
    subtree is successfully reconnected to root.  Until then dentries
    in such subtree are retained only as long as there are references;
    refcount reaching zero means immediate eviction, same as for unhashed
    dentries.  That guarantees that we won't need to hunt them down upon
    umount.
 
 b. A primitive for creation of secondary roots - d_obtain_root(inode).
    Those do _not_ bear DCACHE_DISCONNECTED.  They are placed on the
    per-superblock list (->s_roots), so they can be located at umount
    time for eviction purposes.
 
 c. Helper routines to allocate anonymous dentries, and to help attach
    loose directory dentries at lookup time. They are:
 
     d_obtain_alias(inode) will return a dentry for the given inode.
       If the inode already has a dentry, one of those is returned.
 
       If it doesn't, a new anonymous (IS_ROOT and
       DCACHE_DISCONNECTED) dentry is allocated and attached.
 
       In the case of a directory, care is taken that only one dentry
       can ever be attached.
 
     d_splice_alias(inode, dentry) will introduce a new dentry into the tree;
       either the passed-in dentry or a preexisting alias for the given inode
       (such as an anonymous one created by d_obtain_alias), if appropriate.
       It returns NULL when the passed-in dentry is used, following the calling
       convention of ->lookup.
 
 Filesystem Issues
 -----------------
 
 For a filesystem to be exportable it must:
 
    1. provide the filehandle fragment routines described below.
    2. make sure that d_splice_alias is used rather than d_add
       when ->lookup finds an inode for a given parent and name.
 
       If inode is NULL, d_splice_alias(inode, dentry) is equivalent to::
 
                 d_add(dentry, inode), NULL
 
       Similarly, d_splice_alias(ERR_PTR(err), dentry) = ERR_PTR(err)
 
       Typically the ->lookup routine will simply end with a::
 
                 return d_splice_alias(inode, dentry);
         }
 
 
 
 A file system implementation declares that instances of the filesystem
 are exportable by setting the s_export_op field in the struct
 super_block.  This field must point to a "struct export_operations"
 struct which has the following members:
 
  encode_fh  (optional)
     Takes a dentry and creates a filehandle fragment which can later be used
     to find or create a dentry for the same object.  The default
     implementation creates a filehandle fragment that encodes a 32bit inode
     and generation number for the inode encoded, and if necessary the
     same information for the parent.
 
   fh_to_dentry (mandatory)
     Given a filehandle fragment, this should find the implied object and
     create a dentry for it (possibly with d_obtain_alias).
 
   fh_to_parent (optional but strongly recommended)
     Given a filehandle fragment, this should find the parent of the
     implied object and create a dentry for it (possibly with
     d_obtain_alias).  May fail if the filehandle fragment is too small.
 
   get_parent (optional but strongly recommended)
     When given a dentry for a directory, this should return  a dentry for
     the parent.  Quite possibly the parent dentry will have been allocated
     by d_alloc_anon.  The default get_parent function just returns an error
     so any filehandle lookup that requires finding a parent will fail.
     ->lookup("..") is *not* used as a default as it can leave ".." entries
     in the dcache which are too messy to work with.
 
   get_name (optional)
     When given a parent dentry and a child dentry, this should find a name
     in the directory identified by the parent dentry, which leads to the
     object identified by the child dentry.  If no get_name function is
     supplied, a default implementation is provided which uses vfs_readdir
     to find potential names, and matches inode numbers to find the correct
     match.
 
   flags
     Some filesystems may need to be handled differently than others. The
     export_operations struct also includes a flags field that allows the
     filesystem to communicate such information to nfsd. See the Export
     Operations Flags section below for more explanation.
 
 A filehandle fragment consists of an array of 1 or more 4byte words,
 together with a one byte "type".
 The decode_fh routine should not depend on the stated size that is
 passed to it.  This size may be larger than the original filehandle
 generated by encode_fh, in which case it will have been padded with
 nuls.  Rather, the encode_fh routine should choose a "type" which
 indicates the decode_fh how much of the filehandle is valid, and how
 it should be interpreted.
 
 Export Operations Flags
 -----------------------
 In addition to the operation vector pointers, struct export_operations also
 contains a "flags" field that allows the filesystem to communicate to nfsd
 that it may want to do things differently when dealing with it. The
 following flags are defined:
 
   EXPORT_OP_NOWCC - disable NFSv3 WCC attributes on this filesystem
     RFC 1813 recommends that servers always send weak cache consistency
     (WCC) data to the client after each operation. The server should
     atomically collect attributes about the inode, do an operation on it,
     and then collect the attributes afterward. This allows the client to
     skip issuing GETATTRs in some situations but means that the server
     is calling vfs_getattr for almost all RPCs. On some filesystems
     (particularly those that are clustered or networked) this is expensive
     and atomicity is difficult to guarantee. This flag indicates to nfsd
     that it should skip providing WCC attributes to the client in NFSv3
     replies when doing operations on this filesystem. Consider enabling
     this on filesystems that have an expensive ->getattr inode operation,
     or when atomicity between pre and post operation attribute collection
     is impossible to guarantee.
 
   EXPORT_OP_NOSUBTREECHK - disallow subtree checking on this fs
     Many NFS operations deal with filehandles, which the server must then
     vet to ensure that they live inside of an exported tree. When the
     export consists of an entire filesystem, this is trivial. nfsd can just
     ensure that the filehandle live on the filesystem. When only part of a
     filesystem is exported however, then nfsd must walk the ancestors of the
     inode to ensure that it's within an exported subtree. This is an
     expensive operation and not all filesystems can support it properly.
     This flag exempts the filesystem from subtree checking and causes
     exportfs to get back an error if it tries to enable subtree checking
     on it.
 
   EXPORT_OP_CLOSE_BEFORE_UNLINK - always close cached files before unlinking
     On some exportable filesystems (such as NFS) unlinking a file that
     is still open can cause a fair bit of extra work. For instance,
     the NFS client will do a "sillyrename" to ensure that the file
     sticks around while it's still open. When reexporting, that open
     file is held by nfsd so we usually end up doing a sillyrename, and
     then immediately deleting the sillyrenamed file just afterward when
     the link count actually goes to zero. Sometimes this delete can race
     with other operations (for instance an rmdir of the parent directory).
     This flag causes nfsd to close any open files for this inode _before_
     calling into the vfs to do an unlink or a rename that would replace
     an existing file.

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