9P: introduction of a new cache=mmap model.

 - Add cache=mmap option
 - Make mmap read-write while keeping it as synchronous as possible
 - Build writeback fid on mmap creation if it is writable

Signed-off-by: Dominique Martinet <dominique.martinet@cea.fr>
Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>
diff --git a/fs/9p/v9fs.c b/fs/9p/v9fs.c
index 08f2e1e..14da825 100644
--- a/fs/9p/v9fs.c
+++ b/fs/9p/v9fs.c
@@ -56,7 +56,7 @@
 	/* Options that take no arguments */
 	Opt_nodevmap,
 	/* Cache options */
-	Opt_cache_loose, Opt_fscache,
+	Opt_cache_loose, Opt_fscache, Opt_mmap,
 	/* Access options */
 	Opt_access, Opt_posixacl,
 	/* Error token */
@@ -74,6 +74,7 @@
 	{Opt_cache, "cache=%s"},
 	{Opt_cache_loose, "loose"},
 	{Opt_fscache, "fscache"},
+	{Opt_mmap, "mmap"},
 	{Opt_cachetag, "cachetag=%s"},
 	{Opt_access, "access=%s"},
 	{Opt_posixacl, "posixacl"},
@@ -91,6 +92,9 @@
 	} else if (!strcmp(s, "fscache")) {
 		version = CACHE_FSCACHE;
 		p9_debug(P9_DEBUG_9P, "Cache mode: fscache\n");
+	} else if (!strcmp(s, "mmap")) {
+		version = CACHE_MMAP;
+		p9_debug(P9_DEBUG_9P, "Cache mode: mmap\n");
 	} else if (!strcmp(s, "none")) {
 		version = CACHE_NONE;
 		p9_debug(P9_DEBUG_9P, "Cache mode: none\n");
@@ -220,6 +224,9 @@
 		case Opt_fscache:
 			v9ses->cache = CACHE_FSCACHE;
 			break;
+		case Opt_mmap:
+			v9ses->cache = CACHE_MMAP;
+			break;
 		case Opt_cachetag:
 #ifdef CONFIG_9P_FSCACHE
 			v9ses->cachetag = match_strdup(&args[0]);