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@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Acked-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
Link: https://patch.msgid.link/20250815201712.1745332-2-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 785173aa0739cc5579da30e61b108d8bc9903f0d..25921fbec6856697fc3fffb6bda2a21ba61c43a8 100644 (file)
--- 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 8dd7fbed5a94271f90c45871e009954018ab0b3a..46713b9ece063870767f838e0e488b7ee5a39d05 100644 (file)
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -5024,6 +5024,19 @@ 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);
+
+       if (sk->sk_memcg)
+               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 3f1b62a9fe889ab1ac07f2a210b9de050436b37e..c8a7e4b59db1150610f99d6e599a9d4cf7e137bc 100644 (file)
--- 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)
+               mem_cgroup_sk_inherit(parent, child);
 }
 
 static void mptcp_subflow_ops_override(struct sock *ssk)