linux-kernel - Re: [V2][PATCH] cgroup: fix memory leak caused by missing cgroup_bpf

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Date:   Tue, 19 Oct 2021 19:10:26 +0200
From:   Michal Koutný <mkoutny@...e.com>
To:     Quanyang Wang <quanyang.wang@...driver.com>
Cc:     Ming Lei <ming.lei@...hat.com>, Tejun Heo <tj@...nel.org>,
        Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Jens Axboe <axboe@...nel.dk>, Roman Gushchin <guro@...com>,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: [V2][PATCH] cgroup: fix memory leak caused by missing
 cgroup_bpf_offline

Hi.

On Tue, Oct 19, 2021 at 06:41:14PM +0800, Quanyang Wang <quanyang.wang@...driver.com> wrote:
> So I add 2 "Fixes tags" here to indicate that 2 commits introduce two
> different issues.

AFAIU, both the changes are needed to cause the leak, a single patch
alone won't cause the issue. Is that correct? (Perhaps not as I realize,
see below.)

But on second thought, the problem is the missing percpu_ref_exit() in
the (root) cgroup release path and percpu counter would allocate the
percpu_count_ptr anyway, so 4bfc0bb2c60e is only making the leak more
visible. Is this correct?

I agree the commit 2b0d3d3e4fcf ("percpu_ref: reduce memory footprint of
percpu_ref in fast path") alone did nothing wrong.

[On a related (but independent) note, there seems to be an optimization
opportunity in not dealing with cgroup_bpf at all on the non-default
hierarchies.]

Regards,
Michal