9p: implement optional loose read cache

While cacheing is generally frowned upon in the 9p world, it has its
place -- particularly in situations where the remote file system is
exclusive and/or read-only.  The vacfs views of venti content addressable
store are a real-world instance of such a situation.  To facilitate higher
performance for these workloads (and eventually use the fscache patches),
we have enabled a "loose" cache mode which does not attempt to maintain
any form of consistency on the page-cache or dcache.  This results in over
two orders of magnitude performance improvement for cacheable block reads
in the Bonnie benchmark.  The more aggressive use of the dcache also seems
to improve metadata operational performance.

Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>

Documentation/filesystems/00-INDEX

@@ -4,6 +4,8 @@ Exporting
 	- explanation of how to make filesystems exportable.
 Locking
 	- info on locking rules as they pertain to Linux VFS.
+9p.txt
+	- 9p (v9fs) is an implementation of the Plan 9 remote fs protocol.
 adfs.txt
 	- info and mount options for the Acorn Advanced Disc Filing System.
 afs.txt
@@ -82,8 +84,6 @@ udf.txt
 	- info and mount options for the UDF filesystem.
 ufs.txt
 	- info on the ufs filesystem.
-v9fs.txt
-	- v9fs is a Unix implementation of the Plan 9 9p remote fs protocol.
 vfat.txt
 	- info on using the VFAT filesystem used in Windows NT and Windows 95
 vfs.txt

Documentation/filesystems/9p.txt

@@ -40,6 +40,10 @@ OPTIONS
   aname=name	aname specifies the file tree to access when the server is
   		offering several exported file systems.
 
+  cache=mode	specifies a cacheing policy.  By default, no caches are used.
+			loose = no attempts are made at consistency,
+                                intended for exclusive, read-only mounts
+
   debug=n	specifies debug level.  The debug level is a bitmask.
   			0x01 = display verbose error messages
 			0x02 = developer debug (DEBUG_CURRENT)