Re: [PATCH 1/5] mm: memcg/slab: Fix return child memcg objcg for root memcg

[Roman Gushchin <guro@fb.com>]

On Tue, Oct 27, 2020 at 04:02:52PM +0800, Muchun Song wrote:
> Consider the following memcg hierarchy.
> 
>                     root
>                    /    \
>                   A      B
> 
> If we get the objcg of memcg A failed, the get_obj_cgroup_from_current
> can return the wrong objcg for the root memcg.
> 
> Fixes: bf4f059954dc ("mm: memcg/slab: obj_cgroup API")
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> ---
>  mm/memcontrol.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)

Hi Muchun,

thank you for the patchset!

A generic note: it's usually not a good idea to group fixes with non-fixes
in one patchset if fixes are planned to be backported to stable.

> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 1337775b04f3..fcbd79c5023e 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2945,7 +2945,7 @@ struct mem_cgroup *mem_cgroup_from_obj(void *p)
>  
>  __always_inline struct obj_cgroup *get_obj_cgroup_from_current(void)
>  {
> -	struct obj_cgroup *objcg = NULL;
> +	struct obj_cgroup *objcg;
>  	struct mem_cgroup *memcg;
>  
>  	if (memcg_kmem_bypass())
> @@ -2964,6 +2964,9 @@ __always_inline struct obj_cgroup *get_obj_cgroup_from_current(void)
>  	}
>  	rcu_read_unlock();
>  
> +	if (memcg == root_mem_cgroup)
> +		return NULL;
> +
>  	return objcg;
>  }

I agree, it's a bug. Good catch, thanks!

I would prefer a slightly different fix though. Because we're going to change how
the root memory cgroup is handled (an rfc version posted here:
https://lkml.org/lkml/2020/10/21/612), it's better to not use a comparison
with the root_mem_cgroup and handle the obj_cgroup_tryget() return code instead.
Something like this:

@@ -2973,6 +2973,7 @@ __always_inline struct obj_cgroup *get_obj_cgroup_from_current(void)
 		objcg = rcu_dereference(memcg->objcg);
 		if (objcg && obj_cgroup_tryget(objcg))
 			break;
+		objcg = NULL;
 	}
 	rcu_read_unlock();

Or a more explicit:
   if (obj_cgroup_tryget(objcg)) {
     ...
   } else {
     ...
   }

Also, please, add:
Cc: stable@vger.kernel.org

Thanks!