linux-kernel - Re: [PATCH 1/7] mm: memcontrol: fix page charging in page replacement

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Message-ID: <YHXY123MtiWc5wPW@carbon.dhcp.thefacebook.com>
Date:   Tue, 13 Apr 2021 10:45:59 -0700
From:   Roman Gushchin <guro@...com>
To:     Muchun Song <songmuchun@...edance.com>
CC:     <hannes@...xchg.org>, <mhocko@...nel.org>,
        <akpm@...ux-foundation.org>, <shakeelb@...gle.com>,
        <vdavydov.dev@...il.com>, <linux-kernel@...r.kernel.org>,
        <linux-mm@...ck.org>, <duanxiongchun@...edance.com>,
        <fam.zheng@...edance.com>
Subject: Re: [PATCH 1/7] mm: memcontrol: fix page charging in page replacement

On Tue, Apr 13, 2021 at 02:51:47PM +0800, Muchun Song wrote:
> The pages aren't accounted at the root level, so do not charge the page
> to the root memcg in page replacement. Although we do not display the
> value (mem_cgroup_usage) so there shouldn't be any actual problem, but
> there is a WARN_ON_ONCE in the page_counter_cancel(). Who knows if it
> will trigger? So it is better to fix it.
> 
> Signed-off-by: Muchun Song <songmuchun@...edance.com>
> Acked-by: Johannes Weiner <hannes@...xchg.org>
> Reviewed-by: Shakeel Butt <shakeelb@...gle.com>

Acked-by: Roman Gushchin <guro@...com>

Thanks!
> ---
>  mm/memcontrol.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 64ada9e650a5..f229de925aa5 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -6806,9 +6806,11 @@ void mem_cgroup_migrate(struct page *oldpage, struct page *newpage)
>  	/* Force-charge the new page. The old one will be freed soon */
>  	nr_pages = thp_nr_pages(newpage);
>  
> -	page_counter_charge(&memcg->memory, nr_pages);
> -	if (do_memsw_account())
> -		page_counter_charge(&memcg->memsw, nr_pages);
> +	if (!mem_cgroup_is_root(memcg)) {
> +		page_counter_charge(&memcg->memory, nr_pages);
> +		if (do_memsw_account())
> +			page_counter_charge(&memcg->memsw, nr_pages);
> +	}
>  
>  	css_get(&memcg->css);
>  	commit_charge(newpage, memcg);
> -- 
> 2.11.0
>