linux-kernel - Re: [RFC] cgroup: Avoid thousands of -Wflex-array-member-not-at-end warnings

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Message-ID: <tl6b6chfawtykzrxlmysn6ev7mq7gm764rnlsag7pfme7vhpof@lbwqooaybqmr>
Date: Tue, 2 Sep 2025 13:17:11 +0200
From: Michal Koutný <mkoutny@...e.com>
To: "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Cc: Tejun Heo <tj@...nel.org>, Johannes Weiner <hannes@...xchg.org>, 
	cgroups@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>, 
	linux-hardening@...r.kernel.org, "Gustavo A. R. Silva" <gustavoars@...nel.org>, 
	Chen Ridong <chenridong@...weicloud.com>
Subject: Re: [RFC] cgroup: Avoid thousands of -Wflex-array-member-not-at-end
 warnings

On Tue, Sep 02, 2025 at 09:56:34AM +0200, "Gustavo A. R. Silva" <gustavo@...eddedor.com> wrote:
> If the increase in size is not a problem, then something like this
> works fine (unless there is a problem with moving those two members
> at the end of cgroup_root?):

Please don't forget to tackle cgroup_root allocators. IIUC, this move
towards the end shifts the burden to them.

There's only the rcu_head we care about.

(You seem to be well versed with flex arrays, I was wondering if
something like this could be rearranged to make it work (assuming the
union is at the end of its containers):

	union {
		struct cgroup *ancestors[];
		struct {
			struct cgroup *_root_ancestor;
			struct cgroup *_low_ancestors[];
		};
	};
)

Thanks,
Michal

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