Re: [PATCH v3 net-next 01/12] mptcp: Fix up subflow's memcg when CONFIG_SOCK_CGROUP_DATA=n.

[Kuniyuki Iwashima <kuniyu@google.com>]

From: "Michal Koutný" <mkoutny@suse.com>
Date: Thu, 14 Aug 2025 14:30:05 +0200
> Hello.
> 
> On Tue, Aug 12, 2025 at 05:58:19PM +0000, Kuniyuki Iwashima <kuniyu@google.com> wrote:
> > 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.
> 
> I somewhat remembered
> d752a4986532c ("net: memcg: late association of sock to memcg")
> 
> but IIUC this MPTCP codepath, the socket would never be visible to
> userspace nor manipulated from a proper process context, so there is no
> option to defer the association in similar fashion, correct?
> 
> Then, I wonder whether this isn't a scenario for
> 	o = set_active_memcg(sk->sk_memcg);
> 	newsk =  sk_alloc();
> 	...
> 	set_active_memcg(o);
> 
> i.e. utilize the existing remote charging infra instead of introducing
> specific mem_cgroup_sk_inherit() helper.

Sounds good to me.  sock_create_kern() is much larger than
other set_active_memcg() users, most of which just wrap simple
alloc() functions, but probably that's okay.

I'll use this in the next version.

Thanks!

---8<---
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 8dd7fbed5a94..450862e7fd7a 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -5006,8 +5006,11 @@ void mem_cgroup_sk_alloc(struct sock *sk)
 	if (!in_task())
 		return;
 
+	memcg = current->active_memcg;
+
 	rcu_read_lock();
-	memcg = mem_cgroup_from_task(current);
+	if (likely(!memcg))
+		memcg = mem_cgroup_from_task(current);
 	if (mem_cgroup_is_root(memcg))
 		goto out;
 	if (!cgroup_subsys_on_dfl(memory_cgrp_subsys) && !memcg1_tcpmem_active(memcg))
diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
index 3f1b62a9fe88..a4809054ea6c 100644
--- a/net/mptcp/subflow.c
+++ b/net/mptcp/subflow.c
@@ -1717,14 +1717,6 @@ 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);
@@ -1757,6 +1749,7 @@ int mptcp_subflow_create_socket(struct sock *sk, unsigned short family,
 {
 	struct mptcp_subflow_context *subflow;
 	struct net *net = sock_net(sk);
+	struct mem_cgroup *memcg;
 	struct socket *sf;
 	int err;
 
@@ -1766,7 +1759,9 @@ int mptcp_subflow_create_socket(struct sock *sk, unsigned short family,
 	if (unlikely(!sk->sk_socket))
 		return -EINVAL;
 
+	memcg = set_active_memcg(sk->sk_memcg);
 	err = sock_create_kern(net, family, SOCK_STREAM, IPPROTO_TCP, &sf);
+	set_active_memcg(memcg);
 	if (err)
 		return err;
 
---8<---