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

[Michal Hocko]

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?

> Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> ---
>  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