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

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Date:   Wed, 20 Oct 2021 13:16:01 +0800
From:   Quanyang Wang <quanyang.wang@...driver.com>
To:     Michal Koutný <mkoutny@...e.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 Michal,

On 10/20/21 1:10 AM, Michal Koutný wrote:
> 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.)
Yes, I back to the earlier commit 4bfc0bb2c60e and no memory leak is 
observed.
> 
> 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?
No, the earlier commit 4bfc0bb2c60e introduces a imbalance and the later
commit 2b0d3d3e4fcf introduces a visible leak.

Thanks,
Quanyang
> 
> 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
>