netdev - Re: [PATCH v4 net-next 01/10] mptcp: Fix up subflow's memcg when CONFIG_SOCK_CGROUP

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Message-ID: <CAAVpQUAx9SyA96b_UYofbhM2TPgAGSqq_=-g6ERqmbCZP04-PA@mail.gmail.com>
Date: Thu, 14 Aug 2025 17:05:56 -0700
From: Kuniyuki Iwashima <kuniyu@...gle.com>
To: Shakeel Butt <shakeel.butt@...ux.dev>
Cc: "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>, 
	Andrew Morton <akpm@...ux-foundation.org>, Michal Koutný <mkoutny@...e.com>, 
	Tejun Heo <tj@...nel.org>, 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 v4 net-next 01/10] mptcp: Fix up subflow's memcg when CONFIG_SOCK_CGROUP_DATA=n.

On Thu, Aug 14, 2025 at 4:46 PM Shakeel Butt <shakeel.butt@...ux.dev> wrote:
>
> On Thu, Aug 14, 2025 at 04:27:31PM -0700, Kuniyuki Iwashima wrote:
> > On Thu, Aug 14, 2025 at 2:44 PM Shakeel Butt <shakeel.butt@...ux.dev> wrote:
> > >
> > > On Thu, Aug 14, 2025 at 08:08:33PM +0000, 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 wrap sock_create_kern() for subflow with set_active_memcg()
> > > > using the parent sk->sk_memcg.
> > > >
> > > > Fixes: 3764b0c5651e3 ("mptcp: attach subflow socket to parent cgroup")
> > > > Suggested-by: Michal Koutný <mkoutny@...e.com>
> > > > Signed-off-by: Kuniyuki Iwashima <kuniyu@...gle.com>
> > > > ---
> > > >  mm/memcontrol.c     |  5 ++++-
> > > >  net/mptcp/subflow.c | 11 +++--------
> > > >  2 files changed, 7 insertions(+), 9 deletions(-)
> > > >
> > > > 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;
> > > > +
> > >
> > > Use active_memcg() instead of current->active_memcg and do before the
> > > !in_task() check.
> >
> > Why not reuse the !in_task() check here ?
> > We never use int_active_memcg for socket and also
> > know int_active_memcg is always NULL here.
> >
>
> If we are making mem_cgroup_sk_alloc() work with set_active_memcg()
> infra then make it work for both in_task() and !in_task() contexts.

Considering e876ecc67db80, then I think we should add
set_active_memcg_in_task() and active_memcg_in_task().

or at least we need WARN_ON() if we want to place active_memcg()
before the in_task() check, but this looks ugly.

        memcg = active_memcg();
        if (!in_task() && !memcg)
                return;
        DEBUG_NET_WARN_ON_ONCE(!in_task() && memcg))