fs/9p: allow disable of xattr support on mount

xattr creates a lot of additional messages for 9p in
the current implementation.  This allows users to
conditionalize xattr support on 9p mount if they
are on a connection with bad latency.  Using this
flag is also useful when debugging other aspects
of 9p as it reduces the noise in the trace files.
Signed-off-by: Eric Van Hensbergen <ericvh@kernel.org>
Reviewed-by: Dominique Martinet <asmadeus@codewreck.org>

Documentation/filesystems/9p.rst

@@ -137,6 +137,8 @@ Options
   		This can be used to share devices/named pipes/sockets between
 		hosts.  This functionality will be expanded in later versions.
 
+  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

fs/9p/v9fs.c

@@ -38,7 +38,7 @@ enum {
 	/* String options */
 	Opt_uname, Opt_remotename, Opt_cache, Opt_cachetag,
 	/* Options that take no arguments */
-	Opt_nodevmap,
+	Opt_nodevmap, Opt_noxattr,
 	/* Access options */
 	Opt_access, Opt_posixacl,
 	/* Lock timeout option */
@@ -55,6 +55,7 @@ static const match_table_t tokens = {
 	{Opt_uname, "uname=%s"},
 	{Opt_remotename, "aname=%s"},
 	{Opt_nodevmap, "nodevmap"},
+	{Opt_noxattr, "noxattr"},
 	{Opt_cache, "cache=%s"},
 	{Opt_cachetag, "cachetag=%s"},
 	{Opt_access, "access=%s"},
@@ -149,6 +150,9 @@ int v9fs_show_options(struct seq_file *m, struct dentry *root)
 	if (v9ses->flags & V9FS_POSIX_ACL)
 		seq_puts(m, ",posixacl");
 
+	if (v9ses->flags & V9FS_NO_XATTR)
+		seq_puts(m, ",noxattr");
+
 	return p9_show_client_options(m, v9ses->clnt);
 }
 
@@ -269,6 +273,9 @@ static int v9fs_parse_options(struct v9fs_session_info *v9ses, char *opts)
 		case Opt_nodevmap:
 			v9ses->nodev = 1;
 			break;
+		case Opt_noxattr:
+			v9ses->flags |= V9FS_NO_XATTR;
+			break;
 		case Opt_cachetag:
 #ifdef CONFIG_9P_FSCACHE
 			kfree(v9ses->cachetag);

fs/9p/v9fs.h

@@ -36,7 +36,8 @@ enum p9_session_flags {
 	V9FS_ACCESS_SINGLE	= 0x04,
 	V9FS_ACCESS_USER	= 0x08,
 	V9FS_ACCESS_CLIENT	= 0x10,
-	V9FS_POSIX_ACL		= 0x20
+	V9FS_POSIX_ACL		= 0x20,
+	V9FS_NO_XATTR		= 0x40
 };
 
 /* possible values of ->cache */

fs/9p/vfs_super.c

@@ -64,7 +64,8 @@ v9fs_fill_super(struct super_block *sb, struct v9fs_session_info *v9ses,
 	sb->s_magic = V9FS_MAGIC;
 	if (v9fs_proto_dotl(v9ses)) {
 		sb->s_op = &v9fs_super_ops_dotl;
-		sb->s_xattr = v9fs_xattr_handlers;
+		if (!(v9ses->flags & V9FS_NO_XATTR))
+			sb->s_xattr = v9fs_xattr_handlers;
 	} else {
 		sb->s_op = &v9fs_super_ops;
 		sb->s_time_max = U32_MAX;