mm/vmpressure.c: fix a signedness bug in vmpressure_register_event()

The "mode" and "level" variables are enums and in this context GCC will
treat them as unsigned ints so the error handling is never triggered.

I also removed the bogus initializer because it isn't required any more
and it's sort of confusing.

[akpm@linux-foundation.org: reduce implicit and explicit typecasting]
[akpm@linux-foundation.org: fix return value, add comment, per Matthew]
Link: http://lkml.kernel.org/r/20190925110449.GO3264@mwanda
Fixes: 3cadfa2b ("mm/vmpressure.c: convert to use match_string() helper")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: David Rientjes <rientjes@google.com>
Reviewed-by: Matthew Wilcox <willy@infradead.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Enrico Weigelt <info@metux.net>
Cc: Kate Stewart <kstewart@linuxfoundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/vmpressure.c

@@ -355,6 +355,9 @@ void vmpressure_prio(gfp_t gfp, struct mem_cgroup *memcg, int prio)
  * "hierarchy" or "local").
  *
  * To be used as memcg event method.
+ *
+ * Return: 0 on success, -ENOMEM on memory failure or -EINVAL if @args could
+ * not be parsed.
  */
 int vmpressure_register_event(struct mem_cgroup *memcg,
 			      struct eventfd_ctx *eventfd, const char *args)
@@ -362,7 +365,7 @@ int vmpressure_register_event(struct mem_cgroup *memcg,
 	struct vmpressure *vmpr = memcg_to_vmpressure(memcg);
 	struct vmpressure_event *ev;
 	enum vmpressure_modes mode = VMPRESSURE_NO_PASSTHROUGH;
-	enum vmpressure_levels level = -1;
+	enum vmpressure_levels level;
 	char *spec, *spec_orig;
 	char *token;
 	int ret = 0;
@@ -375,20 +378,18 @@ int vmpressure_register_event(struct mem_cgroup *memcg,
 
 	/* Find required level */
 	token = strsep(&spec, ",");
-	level = match_string(vmpressure_str_levels, VMPRESSURE_NUM_LEVELS, token);
-	if (level < 0) {
-		ret = level;
+	ret = match_string(vmpressure_str_levels, VMPRESSURE_NUM_LEVELS, token);
+	if (ret < 0)
 		goto out;
-	}
+	level = ret;
 
 	/* Find optional mode */
 	token = strsep(&spec, ",");
 	if (token) {
-		mode = match_string(vmpressure_str_modes, VMPRESSURE_NUM_MODES, token);
-		if (mode < 0) {
-			ret = mode;
+		ret = match_string(vmpressure_str_modes, VMPRESSURE_NUM_MODES, token);
+		if (ret < 0)
 			goto out;
-		}
+		mode = ret;
 	}
 
 	ev = kzalloc(sizeof(*ev), GFP_KERNEL);
@@ -404,6 +405,7 @@ int vmpressure_register_event(struct mem_cgroup *memcg,
 	mutex_lock(&vmpr->events_lock);
 	list_add(&ev->node, &vmpr->events);
 	mutex_unlock(&vmpr->events_lock);
+	ret = 0;
 out:
 	kfree(spec_orig);
 	return ret;