Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
From:   Roman Gushchin <guro@fb.com>
To:     Andrew Morton <akpm@linux-foundation.org>, <linux-mm@kvack.org>
CC:     Shakeel Butt <shakeelb@google.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@kernel.org>,
        <linux-kernel@vger.kernel.org>, <kernel-team@fb.com>,
        Roman Gushchin <guro@fb.com>
Subject: [PATCH] mm: memcg/slab: enable slab memory accounting atomically
Date:   Mon, 9 Nov 2020 17:06:15 -0800
Message-ID: <20201110010615.1273043-1-guro@fb.com>
MIME-Version: 1.0
Content-Transfer-Encoding: quoted-printable
Content-Type: text/plain
Precedence: bulk

Many kernel memory accounting paths are guarded by the
memcg_kmem_enabled_key static key. It changes it's state during
the onlining of the first non-root cgroup. However is doesn't
happen atomically: before all call sites will become patched
some charges/uncharges can be skipped, resulting in an unbalanced
charge. The problem is mostly a theoretical issue, unlikely
having a noticeable impact ion the real life.

Before the rework of the slab controller we relied at setting
kmemcg_id after enabling the memcg_kmem_enabled_key static key.
Now we can use the setting of memcg->objcg to enable the slab
memory accounting atomically.

The patch also removes obsolete comments related to already deleted
members of kmem_cache->memcg_params.

Signed-off-by: Roman Gushchin <guro@fb.com>
Fixes: 10befea91b61 ("mm: memcg/slab: use a single set of kmem_caches for=
 all allocations")
---
 include/linux/memcontrol.h | 6 ++----
 mm/memcontrol.c            | 7 ++++---
 2 files changed, 6 insertions(+), 7 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 20108e426f84..01099dfa839c 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -310,7 +310,6 @@ struct mem_cgroup {
 	int			tcpmem_pressure;
=20
 #ifdef CONFIG_MEMCG_KMEM
-        /* Index in the kmem_cache->memcg_params.memcg_caches array */
 	int kmemcg_id;
 	enum memcg_kmem_state kmem_state;
 	struct obj_cgroup __rcu *objcg;
@@ -1641,9 +1640,8 @@ static inline void memcg_kmem_uncharge_page(struct =
page *page, int order)
 }
=20
 /*
- * helper for accessing a memcg's index. It will be used as an index in =
the
- * child cache array in kmem_cache, and also to derive its name. This fu=
nction
- * will return -1 when this is not a kmem-limited memcg.
+ * A helper for accessing memcg's kmem_id, used for getting
+ * corresponding LRU lists.
  */
 static inline int memcg_cache_id(struct mem_cgroup *memcg)
 {
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 69a2893a6455..267cc68fba05 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3675,17 +3675,18 @@ static int memcg_online_kmem(struct mem_cgroup *m=
emcg)
 		memcg_free_cache_id(memcg_id);
 		return -ENOMEM;
 	}
-	objcg->memcg =3D memcg;
-	rcu_assign_pointer(memcg->objcg, objcg);
=20
 	static_branch_enable(&memcg_kmem_enabled_key);
=20
 	/*
 	 * A memory cgroup is considered kmem-online as soon as it gets
-	 * kmemcg_id. Setting the id after enabling static branching will
+	 * objcg. Setting the objcg after enabling static branching will
 	 * guarantee no one starts accounting before all call sites are
 	 * patched.
 	 */
+	objcg->memcg =3D memcg;
+	rcu_assign_pointer(memcg->objcg, objcg);
+
 	memcg->kmemcg_id =3D memcg_id;
 	memcg->kmem_state =3D KMEM_ONLINE;
=20
--=20
2.26.2