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

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Date:   Wed, 24 Mar 2021 12:11:35 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     Roman Gushchin <guro@...com>, Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Vladimir Davydov <vdavydov.dev@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Linux Memory Management List <linux-mm@...ck.org>
Subject: Re: [PATCH] mm: memcontrol: fix memsw uncharge for root_mem_cgroup

On Tue, Mar 23, 2021 at 11:04 PM Muchun Song <songmuchun@...edance.com> 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.
>
> Fixes: 1f47b61fb407 ("mm: memcontrol: fix swap counter leak on swapout from offline cgroup")
> Signed-off-by: Muchun Song <songmuchun@...edance.com>

I am very sorry. I should repent. I suddenly realise the fix is totally
wrong. Because the @memcg cannot be root memcg when
@memcg != @swap_memcg. Please ignore this patch. I am very
sorry for the noise. And sorry to Michal.


> ---
>  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
>