netdev - Re: [PATCH v1 net-next 01/13] mptcp: Fix up subflow's memcg when CONFIG_SOCK_CGROUP

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Message-ID: <CANn89iJQ_CDgAsDQ_cZC=fJJ9Vk5+Yk=RmmS=r3ds9abOqr-nw@mail.gmail.com>
Date: Tue, 22 Jul 2025 07:30:09 -0700
From: Eric Dumazet <edumazet@...gle.com>
To: Kuniyuki Iwashima <kuniyu@...gle.com>
Cc: "David S. Miller" <davem@...emloft.net>, 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>, 
	Simon Horman <horms@...nel.org>, Geliang Tang <geliang@...nel.org>, 
	Muchun Song <muchun.song@...ux.dev>, Kuniyuki Iwashima <kuni1840@...il.com>, netdev@...r.kernel.org, 
	mptcp@...ts.linux.dev, cgroups@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v1 net-next 01/13] mptcp: Fix up subflow's memcg when CONFIG_SOCK_CGROUP_DATA=n.

On Mon, Jul 21, 2025 at 1:36 PM Kuniyuki Iwashima <kuniyu@...gle.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.
>
> 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>