fs/9p: Rework cache modes and add new options to Documentation

Switch cache modes to a bit-mask and use legacy
cache names as shortcuts.  Update documentation to
include information on both shortcuts and bitmasks.

This patch also fixes missing guards related to fscache.

Update the documentation for new mount flags
and cache modes.

Signed-off-by: Eric Van Hensbergen <ericvh@kernel.org>

1 files changed, 37 insertions, 13 deletions

diff --git a/Documentation/filesystems/9p.rst b/Documentation/filesystems/9p.rst
index 0e800b8f73cc..1b5f0cc3e4ca 100644
--- a/Documentation/filesystems/9p.rst
+++ b/Documentation/filesystems/9p.rst
@@ -78,19 +78,39 @@ Options
   		offering several exported file systems.
 
   cache=mode	specifies a caching policy.  By default, no caches are used.
-
-                        none
-				default no cache policy, metadata and data
-                                alike are synchronous.
-			loose
-				no attempts are made at consistency,
-                                intended for exclusive, read-only mounts
-                        fscache
-				use FS-Cache for a persistent, read-only
-				cache backend.
-                        mmap
-				minimal cache that is only used for read-write
-                                mmap.  Northing else is cached, like cache=none
+		The mode can be specified as a bitmask or by using one of the
+		prexisting common 'shortcuts'.
+		The bitmask is described below: (unspecified bits are reserved)
+
+			==========  ====================================================
+			0b00000000  all caches disabled, mmap disabled
+			0b00000001  file caches enabled
+			0b00000010  meta-data caches enabled
+			0b00000100  writeback behavior (as opposed to writethrough)
+			0b00001000  loose caches (no explicit consistency with server)
+			0b10000000  fscache enabled for persistent caching
+			==========  ====================================================
+
+		The current shortcuts and their associated bitmask are:
+
+			=========   ====================================================
+			none        0b00000000 (no caching)
+			readahead   0b00000001 (only read-ahead file caching)
+			mmap        0b00000101 (read-ahead + writeback file cache)
+			loose       0b00001111 (non-coherent file and meta-data caches)
+			fscache     0b10001111 (persistent loose cache)
+			=========   ====================================================
+
+		NOTE: only these shortcuts are tested modes of operation at the
+		moment, so using other combinations of bit-patterns is not
+		known to work.  Work on better cache support is in progress.
+
+		IMPORTANT: loose caches (and by extension at the moment fscache)
+		do not necessarily validate cached values on the server.  In other
+		words changes on the server are not guaranteed to be reflected
+		on the client system.  Only use this mode of operation if you
+		have an exclusive mount and the server will modify the filesystem
+		underneath you.
 
   debug=n	specifies debug level.  The debug level is a bitmask.
 
@@ -137,6 +157,10 @@ Options
   		This can be used to share devices/named pipes/sockets between
 		hosts.  This functionality will be expanded in later versions.
 
+  directio	bypass page cache on all read/write operations
+
+  ignoreqv	ignore qid.version==0 as a marker to ignore cache
+
   noxattr	do not offer xattr functions on this mount.
 
   access	there are four access modes.