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

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