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

[Qi Zheng]

On 11/18/25 7:28 PM, 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@xxxxxxxxxxxxx>
> > >
> > > 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@xxxxxxxxxxxxx>
> > > Signed-off-by: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
> > > ---
> > >   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);
> }

Oh, we can't do that because we still need to consider this objcg when
uncharging. Some pages may be charged before reparenting.

> > >   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@xxxxxxxxx
>
> Thanks for this information, and it seems there's nothing wrong here.
>
> Thanks,
> Qi
>
> >