linux-kernel - Re: [PATCH 1/9] memcg: remove root memcg check from refill

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Message-ID: <fcfbb44b-6ab9-4c39-8a00-3e6200dc25b0@suse.cz>
Date: Tue, 18 Mar 2025 08:59:59 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Shakeel Butt <shakeel.butt@...ux.dev>,
 Andrew Morton <akpm@...ux-foundation.org>
Cc: Johannes Weiner <hannes@...xchg.org>, Michal Hocko <mhocko@...nel.org>,
 Roman Gushchin <roman.gushchin@...ux.dev>,
 Muchun Song <muchun.song@...ux.dev>,
 Sebastian Andrzej Siewior <bigeasy@...utronix.de>, linux-mm@...ck.org,
 cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
 Meta kernel team <kernel-team@...a.com>
Subject: Re: [PATCH 1/9] memcg: remove root memcg check from refill_stock

On 3/15/25 18:49, Shakeel Butt wrote:
> refill_stock can not be called with root memcg, so there is no need to
> check it.
> 
> Signed-off-by: Shakeel Butt <shakeel.butt@...ux.dev>

Acked-by: Vlastimil Babka <vbabka@...e.cz>

It's not trivial to verify this so I'd add
VM_WARN_ON_ONCE(mem_cgroup_is_root(memcg)); as Roman suggested in patch 4 reply.

> ---
>  mm/memcontrol.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index b29433eb17fa..c09a32e93d39 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1883,6 +1883,7 @@ static void __refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
>  		drain_stock(stock);
>  }
>  
> +/* Should never be called with root_mem_cgroup. */
>  static void refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
>  {
>  	unsigned long flags;
> @@ -1892,8 +1893,6 @@ static void refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
>  		 * In case of unlikely failure to lock percpu stock_lock
>  		 * uncharge memcg directly.
>  		 */
> -		if (mem_cgroup_is_root(memcg))
> -			return;
>  		page_counter_uncharge(&memcg->memory, nr_pages);
>  		if (do_memsw_account())
>  			page_counter_uncharge(&memcg->memsw, nr_pages);