linux/Documentation/filesystems/nfs/exporting.rst

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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.