netdev - Re: [PATCH v3 net-next 12/12] net-memcg: Decouple controlled memcg from global protocol memory accounting.

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Message-ID: <20250813130009.GA114408@cmpxchg.org>
Date: Wed, 13 Aug 2025 09:00:09 -0400
From: Johannes Weiner <hannes@...xchg.org>
To: Kuniyuki Iwashima <kuniyu@...gle.com>
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>,
	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>, 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 12/12] net-memcg: Decouple controlled memcg
 from global protocol memory accounting.

On Tue, Aug 12, 2025 at 05:58:30PM +0000, Kuniyuki Iwashima wrote:
> If all workloads were guaranteed to be controlled under memcg, the issue
> could be worked around by setting tcp_mem[0~2] to UINT_MAX.
> 
> In reality, this assumption does not always hold, and processes that
> belong to the root cgroup or opt out of memcg can consume memory up to
> the global limit, becoming a noisy neighbour.

As per the last thread, this is not a supported usecase. Opting out of
memcg coverage for individual cgroups is a self-inflicted problem and
misconfiguration. There is *no* memory isolation *at all* on such
containers. Maybe their socket buffers is the only thing that happens
to matter to *you*, but this is in no way a generic, universal,
upstreamable solution. Knob or auto-detection is not the issue.

Nacked-by: Johannes Weiner <hannes@...xchg.org>