linux-kernel - [PATCH v5 01/11] mm: memcontrol: remove dead code and comments

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Message-Id: <20220530074919.46352-2-songmuchun@bytedance.com>
Date:   Mon, 30 May 2022 15:49:09 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     hannes@...xchg.org, mhocko@...nel.org, roman.gushchin@...ux.dev,
        shakeelb@...gle.com, akpm@...ux-foundation.org
Cc:     cgroups@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, duanxiongchun@...edance.com,
        longman@...hat.com, Muchun Song <songmuchun@...edance.com>
Subject: [PATCH v5 01/11] mm: memcontrol: remove dead code and comments

Since no-hierarchy mode is deprecated after

  commit bef8620cd8e0 ("mm: memcg: deprecate the non-hierarchical mode")

so parent_mem_cgroup() cannot return a NULL except root memcg, however, root
memcg cannot be offline, so it is safe to drop the check of returned value
of parent_mem_cgroup().  Remove those dead code.

The comments in memcg_offline_kmem() above memcg_reparent_list_lrus() are
out of date since

  commit 5abc1e37afa0 ("mm: list_lru: allocate list_lru_one only when needed")

There is no ordering requirement between memcg_reparent_list_lrus() and
memcg_reparent_objcgs(), so remove those outdated comments.

Signed-off-by: Muchun Song <songmuchun@...edance.com>
---
 include/linux/memcontrol.h |  3 +--
 mm/memcontrol.c            | 16 ----------------
 mm/vmscan.c                |  6 +-----
 3 files changed, 2 insertions(+), 23 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 89b14729d59f..0833be256134 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -851,8 +851,7 @@ static inline struct mem_cgroup *lruvec_memcg(struct lruvec *lruvec)
  * parent_mem_cgroup - find the accounting parent of a memcg
  * @memcg: memcg whose parent to find
  *
- * Returns the parent memcg, or NULL if this is the root or the memory
- * controller is in legacy no-hierarchy mode.
+ * Returns the parent memcg, or NULL if this is the root.
  */
 static inline struct mem_cgroup *parent_mem_cgroup(struct mem_cgroup *memcg)
 {
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 598fece89e2b..13da256ff2e4 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3622,17 +3622,7 @@ static void memcg_offline_kmem(struct mem_cgroup *memcg)
 		return;
 
 	parent = parent_mem_cgroup(memcg);
-	if (!parent)
-		parent = root_mem_cgroup;
-
 	memcg_reparent_objcgs(memcg, parent);
-
-	/*
-	 * After we have finished memcg_reparent_objcgs(), all list_lrus
-	 * corresponding to this cgroup are guaranteed to remain empty.
-	 * The ordering is imposed by list_lru_node->lock taken by
-	 * memcg_reparent_list_lrus().
-	 */
 	memcg_reparent_list_lrus(memcg, parent);
 }
 #else
@@ -6593,10 +6583,6 @@ void mem_cgroup_calculate_protection(struct mem_cgroup *root,
 		return;
 
 	parent = parent_mem_cgroup(memcg);
-	/* No parent means a non-hierarchical mode on v1 memcg */
-	if (!parent)
-		return;
-
 	if (parent == root) {
 		memcg->memory.emin = READ_ONCE(memcg->memory.min);
 		memcg->memory.elow = READ_ONCE(memcg->memory.low);
@@ -7050,8 +7036,6 @@ static struct mem_cgroup *mem_cgroup_id_get_online(struct mem_cgroup *memcg)
 			break;
 		}
 		memcg = parent_mem_cgroup(memcg);
-		if (!memcg)
-			memcg = root_mem_cgroup;
 	}
 	return memcg;
 }
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 1678802e03e7..8c6054e06087 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -409,13 +409,9 @@ void reparent_shrinker_deferred(struct mem_cgroup *memcg)
 {
 	int i, nid;
 	long nr;
-	struct mem_cgroup *parent;
+	struct mem_cgroup *parent = parent_mem_cgroup(memcg);
 	struct shrinker_info *child_info, *parent_info;
 
-	parent = parent_mem_cgroup(memcg);
-	if (!parent)
-		parent = root_mem_cgroup;
-
 	/* Prevent from concurrent shrinker_info expand */
 	down_read(&shrinker_rwsem);
 	for_each_node(nid) {
-- 
2.11.0