linux-kernel - Re: [PATCHv2] memcg: fix NULL pointer dereference in __mem_cgroup_usage_unregister

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Message-ID: <20200310111710.GF8447@dhcp22.suse.cz>
Date:   Tue, 10 Mar 2020 12:17:10 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     brookxu <brookxu.cn@...il.com>, hannes@...xchg.org,
        vdavydov.dev@...il.com, akpm@...ux-foundation.org,
        cgroups@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCHv2] memcg: fix NULL pointer dereference in
 __mem_cgroup_usage_unregister_event

On Tue 10-03-20 13:41:49, Kirill A. Shutemov wrote:
> On Tue, Mar 10, 2020 at 10:48:36AM +0100, Michal Hocko wrote:
> > [Cc Kirill, I didn't realize he has implemented this code]
> 
> My first non-trivial mm contribution :P

Everybody has to pay for sins of youth :p

[...]

> > It seems that the code has been broken since 2c488db27b61 ("memcg: clean
> > up memory thresholds"). We've had 371528caec55 ("mm: memcg: Correct
> > unregistring of events attached to the same eventfd") but it didn't
> > catch this case for some reason. Unless I am missing something the code
> > was broken back then already. Kirill please double check after me.
> 
> I think the issue exitsted before 2c488db27b61. The fields had different
> names back then.
> 
> The logic to make unregister never-fail is added in 907860ed381a
> ("cgroups: make cftype.unregister_event() void-returning"). I believe the
> Fixes should point there.

Yes, you seem to be right. It doesn't make a difference much as both
went in to the same kernel but a proper Fixes tag is really valuable.

Thanks for looking into that.

-- 
Michal Hocko
SUSE Labs