dm cache: only allow a single io_mode cache feature to be requested

More than one io_mode feature can be requested when creating a dm cache
device (as is: last one wins).  The io_mode selections are incompatible
with one another, we should force them to be selected exclusively.  Add
a counter to check for more than one io_mode selection.

Fixes: 629d0a8a1a ("dm cache metadata: add "metadata2" feature")
Signed-off-by: John Pittman <jpittman@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-cache-target.c

@@ -2250,7 +2250,7 @@ static int parse_features(struct cache_args *ca, struct dm_arg_set *as,
 		{0, 2, "Invalid number of cache feature arguments"},
 	};
 
-	int r;
+	int r, mode_ctr = 0;
 	unsigned argc;
 	const char *arg;
 	struct cache_features *cf = &ca->features;
@@ -2264,14 +2264,20 @@ static int parse_features(struct cache_args *ca, struct dm_arg_set *as,
 	while (argc--) {
 		arg = dm_shift_arg(as);
 
-		if (!strcasecmp(arg, "writeback"))
+		if (!strcasecmp(arg, "writeback")) {
 			cf->io_mode = CM_IO_WRITEBACK;
+			mode_ctr++;
+		}
 
-		else if (!strcasecmp(arg, "writethrough"))
+		else if (!strcasecmp(arg, "writethrough")) {
 			cf->io_mode = CM_IO_WRITETHROUGH;
+			mode_ctr++;
+		}
 
-		else if (!strcasecmp(arg, "passthrough"))
+		else if (!strcasecmp(arg, "passthrough")) {
 			cf->io_mode = CM_IO_PASSTHROUGH;
+			mode_ctr++;
+		}
 
 		else if (!strcasecmp(arg, "metadata2"))
 			cf->metadata_version = 2;
@@ -2282,6 +2288,11 @@ static int parse_features(struct cache_args *ca, struct dm_arg_set *as,
 		}
 	}
 
+	if (mode_ctr > 1) {
+		*error = "Duplicate cache io_mode features requested";
+		return -EINVAL;
+	}
+
 	return 0;
 }