mm/damon/sysfs: use enum for 'state' input handling

DAMON sysfs 'state' file handling code is using string literals in both
'state_show()' and 'state_store()'.  This makes the code error prone and
inflexible for future extensions.

To improve the situation, this commit defines possible input strings and
'enum' for identifying each input keyword only once, and refactors the
code to reuse those.

Link: https://lkml.kernel.org/r/20220429160606.127307-10-sj@kernel.orgSigned-off-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/damon/sysfs.c

@@ -2053,6 +2053,32 @@ static bool damon_sysfs_ctx_running(struct damon_ctx *ctx)
 	return running;
 }
 
+/*
+ * enum damon_sysfs_cmd - Commands for a specific kdamond.
+ */
+enum damon_sysfs_cmd {
+	/* @DAMON_SYSFS_CMD_ON: Turn the kdamond on. */
+	DAMON_SYSFS_CMD_ON,
+	/* @DAMON_SYSFS_CMD_OFF: Turn the kdamond off. */
+	DAMON_SYSFS_CMD_OFF,
+	/*
+	 * @DAMON_SYSFS_CMD_UPDATE_SCHEMES_STATS: Update scheme stats sysfs
+	 * files.
+	 */
+	DAMON_SYSFS_CMD_UPDATE_SCHEMES_STATS,
+	/*
+	 * @NR_DAMON_SYSFS_CMDS: Total number of DAMON sysfs commands.
+	 */
+	NR_DAMON_SYSFS_CMDS,
+};
+
+/* Should match with enum damon_sysfs_cmd */
+static const char * const damon_sysfs_cmd_strs[] = {
+	"on",
+	"off",
+	"update_schemes_stats",
+};
+
 static ssize_t state_show(struct kobject *kobj, struct kobj_attribute *attr,
 		char *buf)
 {
@@ -2066,7 +2092,9 @@ static ssize_t state_show(struct kobject *kobj, struct kobj_attribute *attr,
 	else
 		running = damon_sysfs_ctx_running(ctx);
 
-	return sysfs_emit(buf, "%s\n", running ? "on" : "off");
+	return sysfs_emit(buf, "%s\n", running ?
+			damon_sysfs_cmd_strs[DAMON_SYSFS_CMD_ON] :
+			damon_sysfs_cmd_strs[DAMON_SYSFS_CMD_OFF]);
 }
 
 static int damon_sysfs_set_attrs(struct damon_ctx *ctx,
@@ -2325,23 +2353,47 @@ static int damon_sysfs_update_schemes_stats(struct damon_sysfs_kdamond *kdamond)
 	return 0;
 }
 
+/*
+ * damon_sysfs_handle_cmd() - Handle a command for a specific kdamond.
+ * @cmd:	The command to handle.
+ * @kdamond:	The kobject wrapper for the associated kdamond.
+ *
+ * This function handles a DAMON sysfs command for a kdamond.
+ *
+ * Return: 0 on success, negative error code otherwise.
+ */
+static int damon_sysfs_handle_cmd(enum damon_sysfs_cmd cmd,
+		struct damon_sysfs_kdamond *kdamond)
+{
+	switch (cmd) {
+	case DAMON_SYSFS_CMD_ON:
+		return damon_sysfs_turn_damon_on(kdamond);
+	case DAMON_SYSFS_CMD_OFF:
+		return damon_sysfs_turn_damon_off(kdamond);
+	case DAMON_SYSFS_CMD_UPDATE_SCHEMES_STATS:
+		return damon_sysfs_update_schemes_stats(kdamond);
+	default:
+		break;
+	}
+	return -EINVAL;
+}
+
 static ssize_t state_store(struct kobject *kobj, struct kobj_attribute *attr,
 		const char *buf, size_t count)
 {
 	struct damon_sysfs_kdamond *kdamond = container_of(kobj,
 			struct damon_sysfs_kdamond, kobj);
-	ssize_t ret;
+	enum damon_sysfs_cmd cmd;
+	ssize_t ret = -EINVAL;
 
 	if (!mutex_trylock(&damon_sysfs_lock))
 		return -EBUSY;
-	if (sysfs_streq(buf, "on"))
-		ret = damon_sysfs_turn_damon_on(kdamond);
-	else if (sysfs_streq(buf, "off"))
-		ret = damon_sysfs_turn_damon_off(kdamond);
-	else if (sysfs_streq(buf, "update_schemes_stats"))
-		ret = damon_sysfs_update_schemes_stats(kdamond);
-	else
-		ret = -EINVAL;
+	for (cmd = 0; cmd < NR_DAMON_SYSFS_CMDS; cmd++) {
+		if (sysfs_streq(buf, damon_sysfs_cmd_strs[cmd])) {
+			ret = damon_sysfs_handle_cmd(cmd, kdamond);
+			break;
+		}
+	}
 	mutex_unlock(&damon_sysfs_lock);
 	if (!ret)
 		ret = count;