mm/vmpressure.c: convert to use match_string() helper

The new helper returns index of the matching string in an array.  We are
going to use it here.

Link: http://lkml.kernel.org/r/20180503203206.44046-1-andriy.shevchenko@linux.intel.comSigned-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/vmpressure.c

@@ -342,26 +342,6 @@ void vmpressure_prio(gfp_t gfp, struct mem_cgroup *memcg, int prio)
 	vmpressure(gfp, memcg, true, vmpressure_win, 0);
 }
 
-static enum vmpressure_levels str_to_level(const char *arg)
-{
-	enum vmpressure_levels level;
-
-	for (level = 0; level < VMPRESSURE_NUM_LEVELS; level++)
-		if (!strcmp(vmpressure_str_levels[level], arg))
-			return level;
-	return -1;
-}
-
-static enum vmpressure_modes str_to_mode(const char *arg)
-{
-	enum vmpressure_modes mode;
-
-	for (mode = 0; mode < VMPRESSURE_NUM_MODES; mode++)
-		if (!strcmp(vmpressure_str_modes[mode], arg))
-			return mode;
-	return -1;
-}
-
 #define MAX_VMPRESSURE_ARGS_LEN	(strlen("critical") + strlen("hierarchy") + 2)
 
 /**
@@ -398,18 +378,18 @@ int vmpressure_register_event(struct mem_cgroup *memcg,
 
 	/* Find required level */
 	token = strsep(&spec, ",");
-	level = str_to_level(token);
-	if (level == -1) {
-		ret = -EINVAL;
+	level = match_string(vmpressure_str_levels, VMPRESSURE_NUM_LEVELS, token);
+	if (level < 0) {
+		ret = level;
 		goto out;
 	}
 
 	/* Find optional mode */
 	token = strsep(&spec, ",");
 	if (token) {
-		mode = str_to_mode(token);
-		if (mode == -1) {
-			ret = -EINVAL;
+		mode = match_string(vmpressure_str_modes, VMPRESSURE_NUM_MODES, token);
+		if (mode < 0) {
+			ret = mode;
 			goto out;
 		}
 	}