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

Harry Yoo
2025-11-17T09:18:47Z

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?

> 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

--
Cheers,
Harry / Hyeonggon