netdev - [PATCH v2 net-next 01/12] mptcp: Fix up subflow's memcg when CONFIG_SOCK_CGROUP

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Message-ID: <20250811173116.2829786-2-kuniyu@google.com>
Date: Mon, 11 Aug 2025 17:30:29 +0000
From: Kuniyuki Iwashima <kuniyu@...gle.com>
To: "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
	Jakub Kicinski <kuba@...nel.org>, Neal Cardwell <ncardwell@...gle.com>, Paolo Abeni <pabeni@...hat.com>, 
	Willem de Bruijn <willemb@...gle.com>, Matthieu Baerts <matttbe@...nel.org>, 
	Mat Martineau <martineau@...nel.org>, Johannes Weiner <hannes@...xchg.org>, 
	Michal Hocko <mhocko@...nel.org>, Roman Gushchin <roman.gushchin@...ux.dev>, 
	Shakeel Butt <shakeel.butt@...ux.dev>, Andrew Morton <akpm@...ux-foundation.org>, 
	"Michal Koutný" <mkoutny@...e.com>, Tejun Heo <tj@...nel.org>
Cc: Simon Horman <horms@...nel.org>, Geliang Tang <geliang@...nel.org>, 
	Muchun Song <muchun.song@...ux.dev>, Mina Almasry <almasrymina@...gle.com>, 
	Kuniyuki Iwashima <kuniyu@...gle.com>, Kuniyuki Iwashima <kuni1840@...il.com>, netdev@...r.kernel.org, 
	mptcp@...ts.linux.dev, cgroups@...r.kernel.org, linux-mm@...ck.org
Subject: [PATCH v2 net-next 01/12] mptcp: Fix up subflow's memcg when CONFIG_SOCK_CGROUP_DATA=n.

When sk_alloc() allocates a socket, mem_cgroup_sk_alloc() sets
sk->sk_memcg based on the current task.

MPTCP subflow socket creation is triggered from userspace or
an in-kernel worker.

In the latter case, sk->sk_memcg is not what we want.  So, we fix
it up from the parent socket's sk->sk_memcg in mptcp_attach_cgroup().

Although the code is placed under #ifdef CONFIG_MEMCG, it is buried
under #ifdef CONFIG_SOCK_CGROUP_DATA.

The two configs are orthogonal.  If CONFIG_MEMCG is enabled without
CONFIG_SOCK_CGROUP_DATA, the subflow's memory usage is not charged
correctly.

Let's move the code out of the wrong ifdef guard.

Note that sk->sk_memcg is freed in sk_prot_free() and the parent
sk holds the refcnt of memcg->css here, so we don't need to use
css_tryget().

Fixes: 3764b0c5651e3 ("mptcp: attach subflow socket to parent cgroup")
Signed-off-by: Kuniyuki Iwashima <kuniyu@...gle.com>
Reviewed-by: Eric Dumazet <edumazet@...gle.com>
---
 include/linux/memcontrol.h |  6 ++++++
 mm/memcontrol.c            | 10 ++++++++++
 net/mptcp/subflow.c        | 11 +++--------
 3 files changed, 19 insertions(+), 8 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 785173aa0739..25921fbec685 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -1604,6 +1604,7 @@ extern struct static_key_false memcg_sockets_enabled_key;
 #define mem_cgroup_sockets_enabled static_branch_unlikely(&memcg_sockets_enabled_key)
 void mem_cgroup_sk_alloc(struct sock *sk);
 void mem_cgroup_sk_free(struct sock *sk);
+void mem_cgroup_sk_inherit(const struct sock *sk, struct sock *newsk);
 
 #if BITS_PER_LONG < 64
 static inline void mem_cgroup_set_socket_pressure(struct mem_cgroup *memcg)
@@ -1661,6 +1662,11 @@ void reparent_shrinker_deferred(struct mem_cgroup *memcg);
 #define mem_cgroup_sockets_enabled 0
 static inline void mem_cgroup_sk_alloc(struct sock *sk) { };
 static inline void mem_cgroup_sk_free(struct sock *sk) { };
+
+static inline void mem_cgroup_sk_inherit(const struct sock *sk, struct sock *newsk)
+{
+}
+
 static inline bool mem_cgroup_under_socket_pressure(struct mem_cgroup *memcg)
 {
 	return false;
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 8dd7fbed5a94..08c6e06750ac 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -5024,6 +5024,16 @@ void mem_cgroup_sk_free(struct sock *sk)
 		css_put(&sk->sk_memcg->css);
 }
 
+void mem_cgroup_sk_inherit(const struct sock *sk, struct sock *newsk)
+{
+	if (sk->sk_memcg == newsk->sk_memcg)
+		return;
+
+	mem_cgroup_sk_free(newsk);
+	css_get(&sk->sk_memcg->css);
+	newsk->sk_memcg = sk->sk_memcg;
+}
+
 /**
  * mem_cgroup_charge_skmem - charge socket memory
  * @memcg: memcg to charge
diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
index 3f1b62a9fe88..6fb635a95baf 100644
--- a/net/mptcp/subflow.c
+++ b/net/mptcp/subflow.c
@@ -1717,19 +1717,14 @@ static void mptcp_attach_cgroup(struct sock *parent, struct sock *child)
 	/* only the additional subflows created by kworkers have to be modified */
 	if (cgroup_id(sock_cgroup_ptr(parent_skcd)) !=
 	    cgroup_id(sock_cgroup_ptr(child_skcd))) {
-#ifdef CONFIG_MEMCG
-		struct mem_cgroup *memcg = parent->sk_memcg;
-
-		mem_cgroup_sk_free(child);
-		if (memcg && css_tryget(&memcg->css))
-			child->sk_memcg = memcg;
-#endif /* CONFIG_MEMCG */
-
 		cgroup_sk_free(child_skcd);
 		*child_skcd = *parent_skcd;
 		cgroup_sk_clone(child_skcd);
 	}
 #endif /* CONFIG_SOCK_CGROUP_DATA */
+
+	if (mem_cgroup_sockets_enabled && parent->sk_memcg)
+		mem_cgroup_sk_inherit(parent, child);
 }
 
 static void mptcp_subflow_ops_override(struct sock *ssk)
-- 
2.51.0.rc0.155.g4a0f42376b-goog