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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.
+
+
+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.