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From:   Muchun Song <songmuchun@bytedance.com>
To:     guro@fb.com, hannes@cmpxchg.org, mhocko@kernel.org,
        akpm@linux-foundation.org, shakeelb@google.com,
        vdavydov.dev@gmail.com
Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        duanxiongchun@bytedance.com, fam.zheng@bytedance.com,
        bsingharora@gmail.com, shy828301@gmail.com,
        alex.shi@linux.alibaba.com, Muchun Song <songmuchun@bytedance.com>
Subject: [RFC PATCH v2 05/18] mm: memcontrol: simplify the logic of objcg pinning memcg
Date:   Fri,  9 Apr 2021 20:29:46 +0800
Message-Id: <20210409122959.82264-6-songmuchun@bytedance.com>
In-Reply-To: <20210409122959.82264-1-songmuchun@bytedance.com>
References: <20210409122959.82264-1-songmuchun@bytedance.com>
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The obj_cgroup_release() and memcg_reparent_objcgs() are serialized by
the css_set_lock. We do not need to care about objcg->memcg being
released in the process of obj_cgroup_release(). So there is no need
to pin memcg before releasing objcg. Remove those pinning logic to
simplfy the code.

There are only two places that modifies the objcg->memcg. One is the
initialization to objcg->memcg in the memcg_online_kmem(), another
is objcgs reparenting in the memcg_reparent_objcgs(). It is also
impossible for the two to run in parallel. So xchg() is unnecessary
and it is enough to use WRITE_ONCE().

Signed-off-by: Muchun Song <songmuchun@bytedance.com>
---
 mm/memcontrol.c | 20 +++++++-------------
 1 file changed, 7 insertions(+), 13 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 1f807448233e..90c1ac58c64c 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -261,7 +261,6 @@ static void obj_cgroup_uncharge_pages(struct obj_cgroup *objcg,
 static void obj_cgroup_release(struct percpu_ref *ref)
 {
 	struct obj_cgroup *objcg = container_of(ref, struct obj_cgroup, refcnt);
-	struct mem_cgroup *memcg;
 	unsigned int nr_bytes;
 	unsigned int nr_pages;
 	unsigned long flags;
@@ -291,11 +290,9 @@ static void obj_cgroup_release(struct percpu_ref *ref)
 	nr_pages = nr_bytes >> PAGE_SHIFT;
 
 	spin_lock_irqsave(&css_set_lock, flags);
-	memcg = obj_cgroup_memcg(objcg);
 	if (nr_pages)
 		obj_cgroup_uncharge_pages(objcg, nr_pages);
 	list_del(&objcg->list);
-	mem_cgroup_put(memcg);
 	spin_unlock_irqrestore(&css_set_lock, flags);
 
 	percpu_ref_exit(ref);
@@ -330,17 +327,14 @@ static void memcg_reparent_objcgs(struct mem_cgroup *memcg,
 
 	spin_lock_irq(&css_set_lock);
 
-	/* Move active objcg to the parent's list */
-	xchg(&objcg->memcg, parent);
-	css_get(&parent->css);
-	list_add(&objcg->list, &parent->objcg_list);
+	/* 1) Ready to reparent active objcg. */
+	list_add(&objcg->list, &memcg->objcg_list);
 
-	/* Move already reparented objcgs to the parent's list */
-	list_for_each_entry(iter, &memcg->objcg_list, list) {
-		css_get(&parent->css);
-		xchg(&iter->memcg, parent);
-		css_put(&memcg->css);
-	}
+	/* 2) Reparent active objcg and already reparented objcgs to parent. */
+	list_for_each_entry(iter, &memcg->objcg_list, list)
+		WRITE_ONCE(iter->memcg, parent);
+
+	/* 3) Move already reparented objcgs to the parent's list */
 	list_splice(&memcg->objcg_list, &parent->objcg_list);
 
 	spin_unlock_irq(&css_set_lock);
-- 
2.11.0