cachefiles: enable on-demand read mode

Enable on-demand read mode by adding an optional parameter to the "bind"
command.

On-demand mode will be turned on when this parameter is "ondemand", i.e.
"bind ondemand". Otherwise cachefiles will work in the original mode.
Signed-off-by: Jeffle Xu <jefflexu@linux.alibaba.com>
Link: https://lore.kernel.org/r/20220509074028.74954-7-jefflexu@linux.alibaba.comAcked-by: David Howells <dhowells@redhat.com>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>

fs/cachefiles/daemon.c

@@ -755,11 +755,6 @@ static int cachefiles_daemon_bind(struct cachefiles_cache *cache, char *args)
 	    cache->brun_percent  >= 100)
 		return -ERANGE;
 
-	if (*args) {
-		pr_err("'bind' command doesn't take an argument\n");
-		return -EINVAL;
-	}
-
 	if (!cache->rootdirname) {
 		pr_err("No cache directory specified\n");
 		return -EINVAL;
@@ -771,6 +766,18 @@ static int cachefiles_daemon_bind(struct cachefiles_cache *cache, char *args)
 		return -EBUSY;
 	}
 
+	if (IS_ENABLED(CONFIG_CACHEFILES_ONDEMAND)) {
+		if (!strcmp(args, "ondemand")) {
+			set_bit(CACHEFILES_ONDEMAND_MODE, &cache->flags);
+		} else if (*args) {
+			pr_err("Invalid argument to the 'bind' command\n");
+			return -EINVAL;
+		}
+	} else if (*args) {
+		pr_err("'bind' command doesn't take an argument\n");
+		return -EINVAL;
+	}
+
 	/* Make sure we have copies of the tag string */
 	if (!cache->tag) {
 		/*