mm: clean up mem_cgroup_iter()

A clean up to make variable names more clear and to improve code
readability.

No functional change.

Link: https://lkml.kernel.org/r/20240905003058.1859929-6-kinseyho@google.comSigned-off-by: Kinsey Ho <kinseyho@google.com>
Reviewed-by: T.J. Mercier <tjmercier@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Michal Koutný <mkoutny@suse.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Tejun Heo <tj@kernel.org>
Cc: Yosry Ahmed <yosryahmed@google.com>
Cc: Zefan Li <lizefan.x@bytedance.com>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/memcontrol.c

@@ -987,8 +987,8 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
 {
 	struct mem_cgroup_reclaim_iter *iter;
 	struct cgroup_subsys_state *css;
-	struct mem_cgroup *memcg;
-	struct mem_cgroup *pos = NULL;
+	struct mem_cgroup *pos;
+	struct mem_cgroup *next;
 
 	if (mem_cgroup_disabled())
 		return NULL;
@@ -998,14 +998,13 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
 
 	rcu_read_lock();
 restart:
-	memcg = NULL;
+	next = NULL;
 
 	if (reclaim) {
 		int gen;
-		struct mem_cgroup_per_node *mz;
+		int nid = reclaim->pgdat->node_id;
 
-		mz = root->nodeinfo[reclaim->pgdat->node_id];
-		iter = &mz->iter;
+		iter = &root->nodeinfo[nid]->iter;
 		gen = atomic_read(&iter->generation);
 
 		/*
@@ -1018,29 +1017,22 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
 			goto out_unlock;
 
 		pos = READ_ONCE(iter->position);
-	} else if (prev) {
+	} else
 		pos = prev;
-	}
 
 	css = pos ? &pos->css : NULL;
 
-	for (;;) {
-		css = css_next_descendant_pre(css, &root->css);
-		if (!css) {
-			break;
-		}
-
+	while ((css = css_next_descendant_pre(css, &root->css))) {
 		/*
 		 * Verify the css and acquire a reference.  The root
 		 * is provided by the caller, so we know it's alive
 		 * and kicking, and don't take an extra reference.
 		 */
-		if (css == &root->css || css_tryget(css)) {
+		if (css == &root->css || css_tryget(css))
 			break;
-		}
 	}
 
-	memcg = mem_cgroup_from_css(css);
+	next = mem_cgroup_from_css(css);
 
 	if (reclaim) {
 		/*
@@ -1048,13 +1040,13 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
 		 * thread, so check that the value hasn't changed since we read
 		 * it to avoid reclaiming from the same cgroup twice.
 		 */
-		if (cmpxchg(&iter->position, pos, memcg) != pos) {
+		if (cmpxchg(&iter->position, pos, next) != pos) {
 			if (css && css != &root->css)
 				css_put(css);
 			goto restart;
 		}
 
-		if (!memcg) {
+		if (!next) {
 			atomic_inc(&iter->generation);
 
 			/*
@@ -1073,7 +1065,7 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
 	if (prev && prev != root)
 		css_put(&prev->css);
 
-	return memcg;
+	return next;
 }
 
 /**