On Tue, Oct 27, 2020 at 10:42:36AM -0700, Roman Gushchin wrote:
> 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@xxxxxxxxxxxxx>
> > ---
> > 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@xxxxxxxxxxxxxxx
After a second thought, it's also a highly theoretical race. In order to have
obj_cgroup_tryget() failed we need to enter get_obj_cgroup_from_current()
racing with cgroup offlining (which is already tricky) and then have
memcg_reparent_objcgs() finish in between reading memcg->objcg and bumping
the reference counter.
So I'm not sure we need a stable@ backport, sorry for the confusion.
Thanks!
