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Date:   Thu, 13 Aug 2020 08:20:49 +0200
From:   Michal Hocko <mhocko@suse.com>
To:     Alex Shi <alex.shi@linux.alibaba.com>
Cc:     akpm@linux-foundation.org, Johannes Weiner <hannes@cmpxchg.org>,
        Vladimir Davydov <vdavydov.dev@gmail.com>,
        cgroups@vger.kernel.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [Resend PATCH 2/6] mm/memcg: remove useless check on
 page->mem_cgroup
Message-ID: <20200813062049.GA9477@dhcp22.suse.cz>
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On Wed 12-08-20 11:25:53, Alex Shi wrote:
> >From 999b0fe5fc65865c3b59ff28500d45572a4a9570 Mon Sep 17 00:00:00 2001
> From: Alex Shi <alex.shi@linux.alibaba.com>
> Date: Wed, 5 Aug 2020 21:02:30 +0800
> Subject: [PATCH 2/6] mm/memcg: bail out early from swap accounting when memcg
>  is disabled
> 
> If we disabled memcg by cgroup_disable=memory, page->memcg will be NULL
> and so the charge is skipped and that will trigger a warning like below.
> Let's return from the funcs earlier.
> 
>  ---[ end trace f1f34bfc3b32ed2f ]---
>  anon flags:0x5005b48008000d(locked|uptodate|dirty|swapbacked)
>  raw: 005005b48008000d dead000000000100 dead000000000122 ffff8897c7c76ad1
>  raw: 0000000000000022 0000000000000000 0000000200000000 0000000000000000
>  page dumped because: VM_WARN_ON_ONCE_PAGE(!memcg)

Yes this is better. It would be even more informative if you added the
backtrace.
 
> Signed-off-by: Alex Shi <alex.shi@linux.alibaba.com>
> Reviewed-by: Roman Gushchin <guro@fb.com>
> Acked-by: Michal Hocko <mhocko@suse.com>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Michal Hocko <mhocko@kernel.org>
> Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: cgroups@vger.kernel.org
> Cc: linux-mm@kvack.org
> Cc: linux-kernel@vger.kernel.org
> ---
>  mm/memcontrol.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 299382fc55a9..419cf565f40b 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -7098,6 +7098,9 @@ void mem_cgroup_swapout(struct page *page, swp_entry_t entry)
>  	VM_BUG_ON_PAGE(PageLRU(page), page);
>  	VM_BUG_ON_PAGE(page_count(page), page);
>  
> +	if (mem_cgroup_disabled())
> +		return;
> +
>  	if (cgroup_subsys_on_dfl(memory_cgrp_subsys))
>  		return;
>  
> @@ -7163,6 +7166,9 @@ int mem_cgroup_try_charge_swap(struct page *page, swp_entry_t entry)
>  	struct mem_cgroup *memcg;
>  	unsigned short oldid;
>  
> +	if (mem_cgroup_disabled())
> +		return 0;
> +
>  	if (!cgroup_subsys_on_dfl(memory_cgrp_subsys))
>  		return 0;
>  
> -- 
> 1.8.3.1

-- 
Michal Hocko
SUSE Labs