Re: [PATCH v1 06/26] mm: memcontrol: return root object cgroup for root memory cgroup

Harry Yoo
2025-11-18T12:12:50Z

On Tue, Nov 18, 2025 at 07:28:41PM +0800, Qi Zheng wrote:
> Hi Harry,
>
> On 11/17/25 5:17 PM, Harry Yoo wrote:
> > On Tue, Oct 28, 2025 at 09:58:19PM +0800, Qi Zheng wrote:
> > > From: Muchun Song <songmuchun@bytedance.com>
> > >
> > > Memory cgroup functions such as get_mem_cgroup_from_folio() and
> > > get_mem_cgroup_from_mm() return a valid memory cgroup pointer,
> > > even for the root memory cgroup. In contrast, the situation for
> > > object cgroups has been different.
> > >
> > > Previously, the root object cgroup couldn't be returned because
> > > it didn't exist. Now that a valid root object cgroup exists, for
> > > the sake of consistency, it's necessary to align the behavior of
> > > object-cgroup-related operations with that of memory cgroup APIs.
> > >
> > > Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> > > Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
> > > ---
> > > include/linux/memcontrol.h | 29 +++++++++++++++++-------
> > > mm/memcontrol.c | 45 ++++++++++++++++++++------------------
> > > mm/percpu.c | 2 +-
> > > 3 files changed, 46 insertions(+), 30 deletions(-)
> > >
> > > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> > > index 6185d8399a54e..9fdbd4970021d 100644
> > > --- a/include/linux/memcontrol.h
> > > +++ b/include/linux/memcontrol.h
> > > @@ -332,6 +332,7 @@ struct mem_cgroup {
> > > #define MEMCG_CHARGE_BATCH 64U
> > > extern struct mem_cgroup *root_mem_cgroup;
> > > +extern struct obj_cgroup *root_obj_cgroup;
> > > enum page_memcg_data_flags {
> > > /* page->memcg_data is a pointer to an slabobj_ext vector */
> > > @@ -549,6 +550,11 @@ static inline bool mem_cgroup_is_root(struct mem_cgroup *memcg)
> > > return (memcg == root_mem_cgroup);
> > > }
> > > +static inline bool obj_cgroup_is_root(const struct obj_cgroup *objcg)
> > > +{
> > > + return objcg == root_obj_cgroup;
> > > +}
> >
> > After reparenting, an objcg may satisfy objcg->memcg == root_mem_cgroup
> > while objcg != root_obj_cgroup. Should they be considered as
> > root objcgs?
>
> Indeed, it's pointless to charge to root_mem_cgroup (objcg->memcg).
>
> So it should be:
>
> static inline bool obj_cgroup_is_root(const struct obj_cgroup *objcg)
> {
> return (objcg == root_obj_cgroup) || (objcg->memcg == root_mem_cgroup);
> }
>

Thanks and tomorrow I'll try to review if will be correct ;)

> > > static inline bool mem_cgroup_disabled(void)
> > > {
> > > return !cgroup_subsys_enabled(memory_cgrp_subsys);
> > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > > index 2afd7f99ca101..d484b632c790f 100644
> > > --- a/mm/memcontrol.c
> > > +++ b/mm/memcontrol.c
> > > @@ -2871,7 +2865,7 @@ int __memcg_kmem_charge_page(struct page *page, gfp_t gfp, int order)
> > > int ret = 0;
> > > objcg = current_obj_cgroup();
> > > - if (objcg) {
> > > + if (!obj_cgroup_is_root(objcg)) {
> >
> > Now that we support the page and slab allocators support allocating memory
> > in NMI contexts (on some archs), current_obj_cgroup() can return NULL
> > if (IS_ENABLED(CONFIG_MEMCG_NMI_UNSAFE) && in_nmi()) returns true
> > (then it leads to a NULL-pointer-deref bug).
> >
> > But IIUC this is applied to kmem charging only (as they use this_cpu ops
> > for stats update), and we don't have to apply the same restriction to
> > charging LRU pages with objcg.
> >
> > Maybe Shakeel has more insight on this.
> >
> > Link: https://lore.kernel.org/all/20250519063142.111219-1-shakeel.butt@linux.dev
>
> Thanks for this information, and it seems there's nothing wrong here.

I mean at least we should not introduce a NULL-pointer-deref bug in
__memcg_kmem_charge_page(), by assuming objcg returned by
current_obj_cgroup() is non-NULL?

1. Someone allocates non-slab kmem in an NMI context (in_nmi() == true),
calling __memcg_kmem_charge_page().
2. current_obj_cgruop() returns NULL because the architectures
has CONFIG_MEMCG_NMI_UNSAFE and it's in an NMI context.
3. obj_cgroup_is_root() returns false since
objcg (NULL) != root_obj_cgroup
4. we pass NULL to obj_cgroup_charge_pages().
5. obj_cgroup_charge_pages() calls get_mem_cgroup_from_objcg(),
dereference objcg->memcg (! a NULL-pointer-deref).

> Thanks,
> Qi
>
> >

--
Cheers,
Harry / Hyeonggon