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From: Michal Hocko <mhocko@suse.com>
To: Miaohe Lin <linmiaohe@huawei.com>
Cc: Roman Gushchin <guro@fb.com>,
	hannes@cmpxchg.org, vdavydov.dev@gmail.com,
	akpm@linux-foundation.org, shakeelb@google.com,
	willy@infradead.org, alexs@kernel.org, richard.weiyang@gmail.com,
	songmuchun@bytedance.com, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org, cgroups@vger.kernel.org
Subject: Re: [PATCH 4/5] mm, memcg: avoid possible NULL pointer dereferencing in mem_cgroup_init()
Date: Mon, 2 Aug 2021 12:42:41 +0200	[thread overview]
Message-ID: <YQfMISKHetFOm1Kx@dhcp22.suse.cz> (raw)
In-Reply-To: <47daf062-f510-edb3-6ec7-f8e7615ad8a0@huawei.com>

On Mon 02-08-21 18:00:10, Miaohe Lin wrote:
> On 2021/8/2 14:43, Michal Hocko wrote:
> > On Sat 31-07-21 10:05:51, Miaohe Lin wrote:
> >> On 2021/7/30 14:44, Michal Hocko wrote:
> >>> On Thu 29-07-21 20:12:43, Roman Gushchin wrote:
> >>>> On Thu, Jul 29, 2021 at 08:57:54PM +0800, Miaohe Lin wrote:
> >>>>> rtpn might be NULL in very rare case. We have better to check it before
> >>>>> dereferencing it. Since memcg can live with NULL rb_tree_per_node in
> >>>>> soft_limit_tree, warn this case and continue.
> >>>>>
> >>>>> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
> >>>>> ---
> >>>>>  mm/memcontrol.c | 2 ++
> >>>>>  1 file changed, 2 insertions(+)
> >>>>>
> >>>>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> >>>>> index 5b4592d1e0f2..70a32174e7c4 100644
> >>>>> --- a/mm/memcontrol.c
> >>>>> +++ b/mm/memcontrol.c
> >>>>> @@ -7109,6 +7109,8 @@ static int __init mem_cgroup_init(void)
> >>>>>  		rtpn = kzalloc_node(sizeof(*rtpn), GFP_KERNEL,
> >>>>>  				    node_online(node) ? node : NUMA_NO_NODE);
> >>>>>  
> >>>>> +		if (WARN_ON_ONCE(!rtpn))
> >>>>> +			continue;
> >>>>
> >>>> I also really doubt that it makes any sense to continue in this case.
> >>>> If this allocations fails (at the very beginning of the system's life, it's an __init function),
> >>>> something is terribly wrong and panic'ing on a NULL-pointer dereference sounds like
> >>>> a perfect choice.
> >>>
> >>> Moreover this is 24B allocation during early boot. Kernel will OOM and
> >>> panic when not being able to find any victim. I do not think we need to
> >>
> >> Agree with you. But IMO it may not be a good idea to leave the rtpn without NULL check. We should defend
> >> it though it could hardly happen. But I'm not insist on this check. I will drop this patch if you insist.
> > 
> > It is not that I would insist. I just do not see any point in the code
> > churn. This check is not going to ever trigger and there is nothing you
> > can do to recover anyway so crashing the kernel is likely the only
> > choice left.
> > 
> 
> I hope I get the point now. What you mean is nothing we can do to recover and panic'ing on a
> NULL-pointer dereference is a perfect choice ? Should we declare that we leave the rtpn without
> NULL check on purpose like below ?
> 
> Many thanks.
> 
> @@ -7109,8 +7109,12 @@ static int __init mem_cgroup_init(void)
>                 rtpn = kzalloc_node(sizeof(*rtpn), GFP_KERNEL,
>                                     node_online(node) ? node : NUMA_NO_NODE);
> 
> -               if (WARN_ON_ONCE(!rtpn))
> -                       continue;
> +               /*
> +                * If this allocation fails (at the very beginning of the
> +                * system's life, it's an __init function), something is
> +                * terribly wrong and panic'ing on a NULL-pointer
> +                * dereference sounds like a perfect choice.
> +                */

I am not really sure this is really worth it. Really we do not really
want to have similar comments all over the early init code, do we?

>                 rtpn->rb_root = RB_ROOT;
>                 rtpn->rb_rightmost = NULL;
>                 spin_lock_init(&rtpn->lock);

-- 
Michal Hocko
SUSE Labs

  reply	other threads:[~2021-08-02 10:42 UTC|newest]

Thread overview: 53+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-29 12:57 [PATCH 0/5] Cleanups and fixup for memcontrol Miaohe Lin
2021-07-29 12:57 ` [PATCH 1/5] mm, memcg: remove unused functions Miaohe Lin
2021-07-29 14:07   ` Shakeel Butt
2021-07-29 14:07     ` Shakeel Butt
2021-07-30  2:39   ` Muchun Song
2021-07-30  2:39     ` Muchun Song
2021-07-30  2:57   ` Roman Gushchin
2021-07-30  6:45   ` Michal Hocko
2021-07-29 12:57 ` [PATCH 2/5] mm, memcg: narrow the scope of percpu_charge_mutex Miaohe Lin
2021-07-30  2:42   ` Muchun Song
2021-07-30  2:42     ` Muchun Song
2021-07-30  3:06   ` Roman Gushchin
2021-07-30  6:50     ` Michal Hocko
2021-07-31  2:29       ` Miaohe Lin
2021-08-02  6:49         ` Michal Hocko
2021-08-02  9:54           ` Miaohe Lin
2021-08-03  3:40         ` Roman Gushchin
2021-08-03  6:29           ` Miaohe Lin
2021-08-03  7:11             ` Michal Hocko
2021-08-03  7:13               ` Roman Gushchin
2021-08-03  7:27                 ` Michal Hocko
2021-08-03  9:33             ` Muchun Song
2021-08-03  9:33               ` Muchun Song
2021-08-03 10:50               ` Miaohe Lin
2021-08-03 14:15       ` Johannes Weiner
2021-08-04  8:20         ` Michal Hocko
2021-08-05  1:44           ` Miaohe Lin
2021-07-30  6:46   ` Michal Hocko
2021-07-29 12:57 ` [PATCH 3/5] mm, memcg: save some atomic ops when flush is already true Miaohe Lin
2021-07-29 14:40   ` Shakeel Butt
2021-07-29 14:40     ` Shakeel Butt
2021-07-30  2:37   ` Muchun Song
2021-07-30  2:37     ` Muchun Song
2021-07-30  3:07   ` Roman Gushchin
2021-07-30  6:51   ` Michal Hocko
2021-07-29 12:57 ` [PATCH 4/5] mm, memcg: avoid possible NULL pointer dereferencing in mem_cgroup_init() Miaohe Lin
2021-07-29 13:52   ` Matthew Wilcox
2021-07-30  1:50     ` Miaohe Lin
2021-07-30  3:12   ` Roman Gushchin
2021-07-30  6:29     ` Miaohe Lin
2021-07-30  6:44     ` Michal Hocko
2021-07-31  2:05       ` Miaohe Lin
2021-08-02  6:43         ` Michal Hocko
2021-08-02 10:00           ` Miaohe Lin
2021-08-02 10:42             ` Michal Hocko [this message]
2021-08-02 11:18               ` Miaohe Lin
2021-07-29 12:57 ` [PATCH 5/5] mm, memcg: always call __mod_node_page_state() with preempt disabled Miaohe Lin
2021-07-29 14:39   ` Shakeel Butt
2021-07-29 14:39     ` Shakeel Butt
2021-07-30  1:52     ` Miaohe Lin
2021-07-30  2:33       ` [External] " Muchun Song
2021-07-30  2:33         ` Muchun Song
2021-07-30  2:46         ` Miaohe Lin

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