Documentation: nfs: convert pnfs.txt to ReST

Convert pnfs.txt to ReST. Content remains mostly unchanged.
Signed-off-by: Daniel W. S. Almeida <dwlsalmeida@gmail.com>
Link: https://lore.kernel.org/r/20200129044917.566906-2-dwlsalmeida@gmail.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/filesystems/index.rst

@@ -51,3 +51,4 @@ Documentation for filesystem implementations.
    overlayfs
    virtiofs
    vfat
+   nfs/index

Documentation/filesystems/nfs/index.rst

+===============================
+NFS
+===============================
+
+
+.. toctree::
+   :maxdepth: 1
+
+   pnfs

Documentation/filesystems/nfs/pnfs.txt → Documentation/filesystems/nfs/pnfs.rst

-Reference counting in pnfs:
+==========================
+Reference counting in pnfs
 ==========================
 
 The are several inter-related caches.  We have layouts which can
@@ -9,7 +10,8 @@ we need to reference count.
 
 
 struct pnfs_layout_hdr
-----------------------
+======================
+
 The on-the-wire command LAYOUTGET corresponds to struct
 pnfs_layout_segment, usually referred to by the variable name lseg.
 Each nfs_inode may hold a pointer to a cache of these layout
@@ -25,7 +27,8 @@ the reference count, as the layout is kept around by the lseg that
 keeps it in the list.
 
 deviceid_cache
---------------
+==============
+
 lsegs reference device ids, which are resolved per nfs_client and
 layout driver type.  The device ids are held in a RCU cache (struct
 nfs4_deviceid_cache).  The cache itself is referenced across each
@@ -38,24 +41,26 @@ justification, but seems reasonable given that we can have multiple
 deviceid's per filesystem, and multiple filesystems per nfs_client.
 
 The hash code is copied from the nfsd code base.  A discussion of
-hashing and variations of this algorithm can be found at:
-http://groups.google.com/group/comp.lang.c/browse_thread/thread/9522965e2b8d3809
+hashing and variations of this algorithm can be found `here.
+<http://groups.google.com/group/comp.lang.c/browse_thread/thread/9522965e2b8d3809>`_
 
 data server cache
------------------
+=================
+
 file driver devices refer to data servers, which are kept in a module
 level cache.  Its reference is held over the lifetime of the deviceid
 pointing to it.
 
 lseg
-----
+====
+
 lseg maintains an extra reference corresponding to the NFS_LSEG_VALID
 bit which holds it in the pnfs_layout_hdr's list.  When the final lseg
 is removed from the pnfs_layout_hdr's list, the NFS_LAYOUT_DESTROYED
 bit is set, preventing any new lsegs from being added.
 
 layout drivers
---------------
+==============
 
 PNFS utilizes what is called layout drivers. The STD defines 4 basic
 layout types: "files", "objects", "blocks", and "flexfiles". For each
@@ -68,6 +73,6 @@ Blocks-layout-driver code is in: fs/nfs/blocklayout/.. directory
 Flexfiles-layout-driver code is in: fs/nfs/flexfilelayout/.. directory
 
 blocks-layout setup
--------------------
+===================
 
 TODO: Document the setup needs of the blocks layout driver