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Date:   Fri, 12 Jun 2020 19:03:52 +0200
From:   Michal =?iso-8859-1?Q?Koutn=FD?= <mkoutny@suse.com>
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     Chris Down <chris@chrisdown.name>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Tejun Heo <tj@kernel.org>, Roman Gushchin <guro@fb.com>,
        linux-mm@kvack.org, cgroups@vger.kernel.org,
        linux-kernel@vger.kernel.org, kernel-team@fb.com
Subject: Re: [PATCH 3/6] mm, memcg: Prevent memory.low load/store tearing
Message-ID: <20200612170352.GA40768@blackbook>
References: <cover.1584034301.git.chris@chrisdown.name>
 <448206f44b0fa7be9dad2ca2601d2bcb2c0b7844.1584034301.git.chris@chrisdown.name>
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Hello.

I see suspicious asymmetry, in the current mainline:
>	WRITE_ONCE(memcg->memory.emin, effective_protection(usage, parent_usage,
>			READ_ONCE(memcg->memory.min),
>			READ_ONCE(parent->memory.emin),
>			atomic_long_read(&parent->memory.children_min_usage)));
>
>	WRITE_ONCE(memcg->memory.elow, effective_protection(usage, parent_usage,
>			memcg->memory.low, READ_ONCE(parent->memory.elow),
>			atomic_long_read(&parent->memory.children_low_usage)));

On Thu, Mar 12, 2020 at 05:33:01PM +0000, Chris Down <chris@chrisdown.name>=
 wrote:
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index aca2964ea494..c85a304fa4a1 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -6262,7 +6262,7 @@ enum mem_cgroup_protection mem_cgroup_protected(str=
uct mem_cgroup *root,
>  		return MEMCG_PROT_NONE;
> =20
>  	emin =3D memcg->memory.min;
> -	elow =3D memcg->memory.low;
> +	elow =3D READ_ONCE(memcg->memory.low);
> =20
>  	parent =3D parent_mem_cgroup(memcg);
>  	/* No parent means a non-hierarchical mode on v1 memcg */
> @@ -6291,7 +6291,7 @@ enum mem_cgroup_protection mem_cgroup_protected(str=
uct mem_cgroup *root,
>  	if (elow && parent_elow) {
>  		unsigned long low_usage, siblings_low_usage;
> =20
> -		low_usage =3D min(usage, memcg->memory.low);
> +		low_usage =3D min(usage, READ_ONCE(memcg->memory.low));
>  		siblings_low_usage =3D atomic_long_read(
>  			&parent->memory.children_low_usage);
Is it possible that these hunks were lost during rebase/merge?

IMHO it should apply as:

-- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -6428,7 +6428,8 @@ enum mem_cgroup_protection mem_cgroup_protected(struc=
t mem_cgroup *root,
                        atomic_long_read(&parent->memory.children_min_usage=
)));

        WRITE_ONCE(memcg->memory.elow, effective_protection(usage, parent_u=
sage,
-                       memcg->memory.low, READ_ONCE(parent->memory.elow),
+                       READ_ONCE(memcg->memory.low),
+                       READ_ONCE(parent->memory.elow),
                        atomic_long_read(&parent->memory.children_low_usage=
)));

 out:


Michal

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