netdev - Re: [PATCH v3 net-next 01/12] mptcp: Fix up subflow's memcg when CONFIG_SOCK_CGROUP

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Message-ID: <603e3360-1f8f-41bd-94fd-a4a94e4a2c34@kernel.org>
Date: Wed, 13 Aug 2025 10:54:19 +0200
From: Matthieu Baerts <matttbe@...nel.org>
To: Kuniyuki Iwashima <kuniyu@...gle.com>,
 "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>,
 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 <kuni1840@...il.com>, netdev@...r.kernel.org,
 mptcp@...ts.linux.dev, cgroups@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v3 net-next 01/12] mptcp: Fix up subflow's memcg when
 CONFIG_SOCK_CGROUP_DATA=n.

Hi Kuniyuki,

On 12/08/2025 19:58, Kuniyuki Iwashima 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.
> 
> 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().

Thank you for the patch!

Acked-by: Matthieu Baerts (NGI0) <matttbe@...nel.org>

Cheers,
Matt
-- 
Sponsored by the NGI0 Core fund.