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X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue, Nov 18, 2025 at 08:11:04PM +0800, Qi Zheng wrote: > > > 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 > > > > > > > > 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 > > > > Signed-off-by: Qi Zheng > > > > --- > > > >   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. Ouch, right. We don't know if it's charged before reparenting and so it can break statistics in a few places if we skip uncharging it after repareting. And I think we don't charge new pages to objcgs that satisfy (objcg->memcg == root_mem_cgroup) && (objcg != root_obj_cgroup) after they're reparented anyway... -- Cheers, Harry / Hyeonggon