linux-kernel - Re: [PATCH] mm: memcontrol: fix memsw uncharge for root_mem

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Message-ID: <YFoTFJ349TqzYx40@dhcp22.suse.cz>
Date:   Tue, 23 Mar 2021 17:11:00 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Muchun Song <songmuchun@...edance.com>
Cc:     guro@...com, hannes@...xchg.org, akpm@...ux-foundation.org,
        shakeelb@...gle.com, vdavydov.dev@...il.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        duanxiongchun@...edance.com
Subject: Re: [PATCH] mm: memcontrol: fix memsw uncharge for root_mem_cgroup

On Tue 23-03-21 22:56:53, Muchun Song wrote:
> The pages aren't accounted at the root level, so we cannot uncharge the
> page to the memsw counter for the root memcg. Fix this.

The patch is correct but I do wonder whether this matters much in the
end. We shouldn't really rely on a correct page counter for the root
memcg AFAICS in the kernel. We do not display the value
(mem_cgroup_usage) so there shouldn't be any actual problem. Unless I am
missing something make sure to spell that out in the changelog.

> Fixes: 1f47b61fb407 ("mm: memcontrol: fix swap counter leak on swapout from offline cgroup")
> Signed-off-by: Muchun Song <songmuchun@...edance.com>

Acked-by: Michal Hocko <mhocko@...e.com>
> ---
>  mm/memcontrol.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 533b4b31b464..7d765a106684 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -7155,7 +7155,8 @@ void mem_cgroup_swapout(struct page *page, swp_entry_t entry)
>  	if (!cgroup_memory_noswap && memcg != swap_memcg) {
>  		if (!mem_cgroup_is_root(swap_memcg))
>  			page_counter_charge(&swap_memcg->memsw, nr_entries);
> -		page_counter_uncharge(&memcg->memsw, nr_entries);
> +		if (!mem_cgroup_is_root(memcg))
> +			page_counter_uncharge(&memcg->memsw, nr_entries);
>  	}
>  
>  	/*
> -- 
> 2.11.0

-- 
Michal Hocko
SUSE Labs