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Date:   Thu, 2 Jul 2020 18:22:02 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     Roman Gushchin <guro@fb.com>
Cc:     Naresh Kamboju <naresh.kamboju@linaro.org>,
        Shakeel Butt <shakeelb@google.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
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        open list <linux-kernel@vger.kernel.org>,
        lkft-triage@lists.linaro.org, Chris Down <chris@chrisdown.name>
Subject: Re: BUG: Bad page state in process - page dumped because: page still
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On Wed 01-07-20 11:45:52, Roman Gushchin wrote:
[...]
> >From c97afecd32c0db5e024be9ba72f43d22974f5bcd Mon Sep 17 00:00:00 2001
> From: Roman Gushchin <guro@fb.com>
> Date: Wed, 1 Jul 2020 11:05:32 -0700
> Subject: [PATCH] mm: kmem: make memcg_kmem_enabled() irreversible
> 
> Historically the kernel memory accounting was an opt-in feature, which
> could be enabled for individual cgroups. But now it's not true, and
> it's on by default both on cgroup v1 and cgroup v2.  And as long as a
> user has at least one non-root memory cgroup, the kernel memory
> accounting is on. So in most setups it's either always on (if memory
> cgroups are in use and kmem accounting is not disabled), either always
> off (otherwise).
> 
> memcg_kmem_enabled() is used in many places to guard the kernel memory
> accounting code. If memcg_kmem_enabled() can reverse from returning
> true to returning false (as now), we can't rely on it on release paths
> and have to check if it was on before.
> 
> If we'll make memcg_kmem_enabled() irreversible (always returning true
> after returning it for the first time), it'll make the general logic
> more simple and robust. It also will allow to guard some checks which
> otherwise would stay unguarded.
> 
> Signed-off-by: Roman Gushchin <guro@fb.com>
> ---
>  mm/memcontrol.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 50ae77f3985e..2d018a51c941 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -3582,7 +3582,8 @@ static int memcg_online_kmem(struct mem_cgroup *memcg)
>  	objcg->memcg = memcg;
>  	rcu_assign_pointer(memcg->objcg, objcg);
>  
> -	static_branch_inc(&memcg_kmem_enabled_key);
> +	if (!memcg_kmem_enabled())
> +		static_branch_inc(&memcg_kmem_enabled_key);

Wouldn't be static_branch_enable() more readable?

>  	/*
>  	 * A memory cgroup is considered kmem-online as soon as it gets
>  	 * kmemcg_id. Setting the id after enabling static branching will
> @@ -3643,9 +3644,6 @@ static void memcg_free_kmem(struct mem_cgroup *memcg)
>  	/* css_alloc() failed, offlining didn't happen */
>  	if (unlikely(memcg->kmem_state == KMEM_ONLINE))
>  		memcg_offline_kmem(memcg);
> -
> -	if (memcg->kmem_state == KMEM_ALLOCATED)
> -		static_branch_dec(&memcg_kmem_enabled_key);
>  }
>  #else
>  static int memcg_online_kmem(struct mem_cgroup *memcg)
> -- 
> 2.26.2

-- 
Michal Hocko
SUSE Labs