Re: [PATCH v2] mm: memcontrol: switch to rcu protection in drain_all_stock()

[Michal Hocko]

On Fri 02-08-19 12:22:41, Roman Gushchin wrote:
> Commit 72f0184c8a00 ("mm, memcg: remove hotplug locking from try_charge")
> introduced css_tryget()/css_put() calls in drain_all_stock(),
> which are supposed to protect the target memory cgroup from being
> released during the mem_cgroup_is_descendant() call.
> 
> However, it's not completely safe. In theory, memcg can go away
> between reading stock->cached pointer and calling css_tryget().
> 
> This can happen if drain_all_stock() races with drain_local_stock()
> performed on the remote cpu as a result of a work, scheduled
> by the previous invocation of drain_all_stock().

Maybe I am still missing something but I do not see how 72f0184c8a00
changed the existing race. get_online_cpus doesn't prevent the same race
right? If this is the case then it would be great to clarify that. I
know that you are mostly after clarifying that css_tryget is
insufficient but the above sounds like 72f0184c8a00 has introduced a
regression.

> The race is a bit theoretical and there are few chances to trigger
> it, but the current code looks a bit confusing, so it makes sense
> to fix it anyway. The code looks like as if css_tryget() and
> css_put() are used to protect stocks drainage. It's not necessary
> because stocked pages are holding references to the cached cgroup.
> And it obviously won't work for works, scheduled on other cpus.
> 
> So, let's read the stock->cached pointer and evaluate the memory
> cgroup inside a rcu read section, and get rid of
> css_tryget()/css_put() calls.
> 
> v2: added some explanations to the commit message, no code changes
> 
> Signed-off-by: Roman Gushchin <guro@xxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: Hillf Danton <hdanton@xxxxxxxx>

Other than that.
Acked-by: Michal Hocko <mhocko@xxxxxxxx>

> ---
>  mm/memcontrol.c | 17 +++++++++--------
>  1 file changed, 9 insertions(+), 8 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 5c7b9facb0eb..d856b64426b7 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2235,21 +2235,22 @@ static void drain_all_stock(struct mem_cgroup *root_memcg)
>  	for_each_online_cpu(cpu) {
>  		struct memcg_stock_pcp *stock = &per_cpu(memcg_stock, cpu);
>  		struct mem_cgroup *memcg;
> +		bool flush = false;
>  
> +		rcu_read_lock();
>  		memcg = stock->cached;
> -		if (!memcg || !stock->nr_pages || !css_tryget(&memcg->css))
> -			continue;
> -		if (!mem_cgroup_is_descendant(memcg, root_memcg)) {
> -			css_put(&memcg->css);
> -			continue;
> -		}
> -		if (!test_and_set_bit(FLUSHING_CACHED_CHARGE, &stock->flags)) {
> +		if (memcg && stock->nr_pages &&
> +		    mem_cgroup_is_descendant(memcg, root_memcg))
> +			flush = true;
> +		rcu_read_unlock();
> +
> +		if (flush &&
> +		    !test_and_set_bit(FLUSHING_CACHED_CHARGE, &stock->flags)) {
>  			if (cpu == curcpu)
>  				drain_local_stock(&stock->work);
>  			else
>  				schedule_work_on(cpu, &stock->work);
>  		}
> -		css_put(&memcg->css);
>  	}
>  	put_cpu();
>  	mutex_unlock(&percpu_charge_mutex);
> -- 
> 2.21.0

-- 
Michal Hocko
SUSE Labs