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

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Date:   Fri, 12 Feb 2021 10:56:54 -0800
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Muchun Song <songmuchun@...edance.com>,
        Huang Ying <ying.huang@...el.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Cgroups <cgroups@...r.kernel.org>, Linux MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/4] mm: memcontrol: fix swap uncharge on cgroup v2

CCing more folks.

On Fri, Feb 12, 2021 at 9:14 AM Muchun Song <songmuchun@...edance.com> 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.
>
> Fixes: 2d1c498072de ("mm: memcontrol: make swap tracking an integral part of memory control")
> Signed-off-by: Muchun Song <songmuchun@...edance.com>

What's the user visible impact of this change?

One impact I can see is that without this patch meminfo's (SwapTotal -
SwapFree) is larger than the sum of top level memory.swap.current.
This change will reduce that gap.

BTW what about per-cpu slots_ret cache? Should we call
mem_cgroup_uncharge_swap() before putting in the cache after this
change?

> ---
>  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
>