Re: [PATCH 3/6] mm, memcg: Prevent memory.low load/store tearing

["Michal Koutný" <mkoutny-IBi9RG/b67k@public.gmane.org>]

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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-6Bi1550iOqEnzZ6mRAm98g@public.gmane.org> 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(struct mem_cgroup *root,
>  		return MEMCG_PROT_NONE;
>  
>  	emin = memcg->memory.min;
> -	elow = memcg->memory.low;
> +	elow = READ_ONCE(memcg->memory.low);
>  
>  	parent = 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(struct mem_cgroup *root,
>  	if (elow && parent_elow) {
>  		unsigned long low_usage, siblings_low_usage;
>  
> -		low_usage = min(usage, memcg->memory.low);
> +		low_usage = min(usage, READ_ONCE(memcg->memory.low));
>  		siblings_low_usage = 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(struct mem_cgroup *root,
                        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),
+                       READ_ONCE(memcg->memory.low),
+                       READ_ONCE(parent->memory.elow),
                        atomic_long_read(&parent->memory.children_low_usage)));

 out:


Michal

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