We never charge the page counters of root memcg, so there is no need to
put root memcg in the memcg stock. At the moment, refill_stock() can be
called from try_charge_memcg(), obj_cgroup_uncharge_pages() and
mem_cgroup_uncharge_skmem().

The try_charge_memcg() and mem_cgroup_uncharge_skmem() are never called
with root memcg, so those are fine. However obj_cgroup_uncharge_pages()
can potentially call refill_stock() with root memcg if the objcg object
has been reparented over to the root memcg. Let's just avoid
refill_stock() from obj_cgroup_uncharge_pages() for root memcg.

Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
---
 mm/memcontrol.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index e08ce52caabd..393b73aec6dd 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2643,7 +2643,8 @@ static void obj_cgroup_uncharge_pages(struct obj_cgroup *objcg,
 
 	mod_memcg_state(memcg, MEMCG_KMEM, -nr_pages);
 	memcg1_account_kmem(memcg, -nr_pages);
-	refill_stock(memcg, nr_pages);
+	if (!mem_cgroup_is_root(memcg))
+		refill_stock(memcg, nr_pages);
 
 	css_put(&memcg->css);
 }
-- 
2.47.1

On Wed, Mar 12, 2025 at 10:48:12PM -0700, Shakeel Butt wrote:
> We never charge the page counters of root memcg, so there is no need to
> put root memcg in the memcg stock. At the moment, refill_stock() can be
> called from try_charge_memcg(), obj_cgroup_uncharge_pages() and
> mem_cgroup_uncharge_skmem().
> 
> The try_charge_memcg() and mem_cgroup_uncharge_skmem() are never called
> with root memcg, so those are fine. However obj_cgroup_uncharge_pages()
> can potentially call refill_stock() with root memcg if the objcg object
> has been reparented over to the root memcg. Let's just avoid
> refill_stock() from obj_cgroup_uncharge_pages() for root memcg.
> 
> Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>

Acked-by: Johannes Weiner <hannes@cmpxchg.org>

On Wed 12-03-25 22:48:12, Shakeel Butt wrote:
> We never charge the page counters of root memcg, so there is no need to
> put root memcg in the memcg stock. At the moment, refill_stock() can be
> called from try_charge_memcg(), obj_cgroup_uncharge_pages() and
> mem_cgroup_uncharge_skmem().
> 
> The try_charge_memcg() and mem_cgroup_uncharge_skmem() are never called
> with root memcg, so those are fine. However obj_cgroup_uncharge_pages()
> can potentially call refill_stock() with root memcg if the objcg object
> has been reparented over to the root memcg. Let's just avoid
> refill_stock() from obj_cgroup_uncharge_pages() for root memcg.

Makes sense. It is just pointless draining of a different memcg.

> Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>

Acked-by: Michal Hocko <mhockoc@suse.com>
Thanks!

> ---
>  mm/memcontrol.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index e08ce52caabd..393b73aec6dd 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2643,7 +2643,8 @@ static void obj_cgroup_uncharge_pages(struct obj_cgroup *objcg,
>  
>  	mod_memcg_state(memcg, MEMCG_KMEM, -nr_pages);
>  	memcg1_account_kmem(memcg, -nr_pages);
> -	refill_stock(memcg, nr_pages);
> +	if (!mem_cgroup_is_root(memcg))
> +		refill_stock(memcg, nr_pages);
>  
>  	css_put(&memcg->css);
>  }
> -- 
> 2.47.1
> 

-- 
Michal Hocko
SUSE Labs