Re: [External] Re: [PATCH] mm/memcg: fix NULL pointer dereference at workingset_eviction

[Muchun Song <songmuchun@bytedance.com>]

On Mon, Nov 30, 2020 at 9:23 PM Michal Hocko <mhocko@suse.com> wrote:
>
> On Mon 30-11-20 21:15:12, Muchun Song wrote:
> > We found a case of kernel panic. The stack trace is as follows
> > (omit some irrelevant information):
> >
> >     BUG: kernel NULL pointer dereference, address: 00000000000000c8
> >     RIP: 0010:workingset_eviction+0x26b/0x450
> >     Call Trace:
> >      __remove_mapping+0x224/0x2b0
> >      shrink_page_list+0x8c2/0x14e0
> >      shrink_inactive_list+0x1bf/0x3f0
> >      ? do_raw_spin_unlock+0x49/0xc0
> >      ? _raw_spin_unlock+0xa/0x20
> >      shrink_lruvec+0x401/0x640
> >
> > This was caused by commit 76761ffa9ea1 ("mm/memcg: bail out early when
> > !memcg in mem_cgroup_lruvec"). When the parameter of memcg is NULL, we
> > should not use the &pgdat->__lruvec. So this just reverts commit
> > 76761ffa9ea1 to fix it.
> >
> > Fixes: 76761ffa9ea1 ("mm/memcg: bail out early when !memcg in mem_cgroup_lruvec")
>
> I do not see any commits like that in the current Linus tree. Is this a
> commit id from the linux-next? If yes, can we just fold it into the
> respective patch in mmotm tree please?

Yes. This commit is on the linux-next tree. Of course can.

>
> > Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> > ---
> >  include/linux/memcontrol.h | 15 +++++++++------
> >  1 file changed, 9 insertions(+), 6 deletions(-)
> >
> > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> > index f9a496c4eac7..a1416205507c 100644
> > --- a/include/linux/memcontrol.h
> > +++ b/include/linux/memcontrol.h
> > @@ -610,17 +610,20 @@ mem_cgroup_nodeinfo(struct mem_cgroup *memcg, int nid)
> >  static inline struct lruvec *mem_cgroup_lruvec(struct mem_cgroup *memcg,
> >                                              struct pglist_data *pgdat)
> >  {
> > +     struct mem_cgroup_per_node *mz;
> >       struct lruvec *lruvec;
> >
> > -     if (mem_cgroup_disabled() || !memcg) {
> > +     if (mem_cgroup_disabled()) {
> >               lruvec = &pgdat->__lruvec;
> > -     } else {
> > -             struct mem_cgroup_per_node *mz;
> > -
> > -             mz = mem_cgroup_nodeinfo(memcg, pgdat->node_id);
> > -             lruvec = &mz->lruvec;
> > +             goto out;
> >       }
> >
> > +     if (!memcg)
> > +             memcg = root_mem_cgroup;
> > +
> > +     mz = mem_cgroup_nodeinfo(memcg, pgdat->node_id);
> > +     lruvec = &mz->lruvec;
> > +out:
> >       /*
> >        * Since a node can be onlined after the mem_cgroup was created,
> >        * we have to be prepared to initialize lruvec->pgdat here;
> > --
> > 2.11.0
> >
>
> --
> Michal Hocko
> SUSE Labs



-- 
Yours,
Muchun