linux-kernel - Re: [PATCH 4/4] mm: memcontrol: fix swap uncharge on cgroup v2

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Message-ID: <YCpDSnLSDoE/FHK5@dhcp22.suse.cz>
Date:   Mon, 15 Feb 2021 10:47:54 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Muchun Song <songmuchun@...edance.com>
Cc:     hannes@...xchg.org, vdavydov.dev@...il.com,
        akpm@...ux-foundation.org, cgroups@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/4] mm: memcontrol: fix swap uncharge on cgroup v2

On Sat 13-02-21 01:01:59, Muchun Song wrote:
> The swap charges the actual number of swap entries on cgroup v2.
> If a swap cache page is charged successful, and then we uncharge
> the swap counter. It is wrong on cgroup v2. Because the swap
> entry is not freed.

Is there any actual problem though? Can you describe the specific
scenario please? Swap cache charge life time is a bit tricky and I have
to confess I have to relearn it every time I need to understand it. The
patch would be much more easier to review if the changelog was much more
specific.

> Fixes: 2d1c498072de ("mm: memcontrol: make swap tracking an integral part of memory control")
> Signed-off-by: Muchun Song <songmuchun@...edance.com>
> ---
>  mm/memcontrol.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index c737c8f05992..be6bc5044150 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -6753,7 +6753,7 @@ int mem_cgroup_charge(struct page *page, struct mm_struct *mm, gfp_t gfp_mask)
>  	memcg_check_events(memcg, page);
>  	local_irq_enable();
>  
> -	if (PageSwapCache(page)) {
> +	if (!cgroup_subsys_on_dfl(memory_cgrp_subsys) && PageSwapCache(page)) {
>  		swp_entry_t entry = { .val = page_private(page) };
>  		/*
>  		 * The swap entry might not get freed for a long time,
> -- 
> 2.11.0

-- 
Michal Hocko
SUSE Labs