Merge tag '9p-6.4-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/ericvh/v9fs

Pull 9p updates from Eric Van Hensbergen:
 "This includes a number of patches that didn't quite make the cut last
  merge window while we addressed some outstanding issues and review
  comments. It includes some new caching modes for those that only want
  readahead caches and reworks how we do writeback caching so we are not
  keeping extra references around which both causes performance problems
  and uses lots of additional resources on the server.

  It also includes a new flag to force disabling of xattrs which can
  also cause major performance issues, particularly if the underlying
  filesystem on the server doesn't support them.

  Finally it adds a couple of additional mount options to better support
  directio and enabling caches when the server doesn't support
  qid.version.

  There was one late-breaking bug report that has also been included as
  its own patch where I forgot to propagate an embarassing bit-logic fix
  to the various variations of open"

* tag '9p-6.4-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/ericvh/v9fs:
  fs/9p: Fix bit operation logic error
  fs/9p: Rework cache modes and add new options to Documentation
  fs/9p: remove writeback fid and fix per-file modes
  fs/9p: Add new mount modes
  9p: Add additional debug flags and open modes
  fs/9p: allow disable of xattr support on mount
  fs/9p: Remove unnecessary superblock flags
  fs/9p: Consolidate file operations and add readahead and writeback

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.
+		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)
 
-                        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
+			==========  ====================================================
+			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 +157,12 @@ 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.
 			user
 				if a user tries to access a file on v9fs