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

[Michal Hocko]

On Thu 12-03-20 17:33:01, Chris Down wrote:
> This can be set concurrently with reads, which may cause the wrong value
> to be propagated.
> 
> Signed-off-by: Chris Down <chris@xxxxxxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Roman Gushchin <guro@xxxxxx>
> Cc: Tejun Heo <tj@xxxxxxxxxx>
> Cc: linux-mm@xxxxxxxxx
> Cc: cgroups@xxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Cc: kernel-team@xxxxxx

Acked-by: Michal Hocko <mhocko@xxxxxxxx>

> ---
>  mm/memcontrol.c   | 4 ++--
>  mm/page_counter.c | 9 ++++++---
>  2 files changed, 8 insertions(+), 5 deletions(-)
> 
> 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);
>  
> diff --git a/mm/page_counter.c b/mm/page_counter.c
> index 50184929b61f..18b7f779f2e2 100644
> --- a/mm/page_counter.c
> +++ b/mm/page_counter.c
> @@ -17,6 +17,7 @@ static void propagate_protected_usage(struct page_counter *c,
>  				      unsigned long usage)
>  {
>  	unsigned long protected, old_protected;
> +	unsigned long low;
>  	long delta;
>  
>  	if (!c->parent)
> @@ -34,8 +35,10 @@ static void propagate_protected_usage(struct page_counter *c,
>  			atomic_long_add(delta, &c->parent->children_min_usage);
>  	}
>  
> -	if (c->low || atomic_long_read(&c->low_usage)) {
> -		if (usage <= c->low)
> +	low = READ_ONCE(c->low);
> +
> +	if (low || atomic_long_read(&c->low_usage)) {
> +		if (usage <= low)
>  			protected = usage;
>  		else
>  			protected = 0;
> @@ -231,7 +234,7 @@ void page_counter_set_low(struct page_counter *counter, unsigned long nr_pages)
>  {
>  	struct page_counter *c;
>  
> -	counter->low = nr_pages;
> +	WRITE_ONCE(counter->low, nr_pages);
>  
>  	for (c = counter; c; c = c->parent)
>  		propagate_protected_usage(c, atomic_long_read(&c->usage));
> -- 
> 2.25.1
> 

-- 
Michal Hocko
SUSE Labs