linux-kernel - Re: [PATCH 4/5] mm, memcg: avoid possible NULL pointer dereferencing in mem_cgroup

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Date:   Fri, 30 Jul 2021 08:44:33 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Roman Gushchin <guro@...com>
Cc:     Miaohe Lin <linmiaohe@...wei.com>, hannes@...xchg.org,
        vdavydov.dev@...il.com, akpm@...ux-foundation.org,
        shakeelb@...gle.com, willy@...radead.org, alexs@...nel.org,
        richard.weiyang@...il.com, songmuchun@...edance.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        cgroups@...r.kernel.org
Subject: Re: [PATCH 4/5] mm, memcg: avoid possible NULL pointer dereferencing
 in mem_cgroup_init()

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@...wei.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
do any special handling here.
-- 
Michal Hocko
SUSE Labs