Re: [PATCH v3 4/5] mm: memcg: charge memcg percpu memory to the parent cgroup

[Johannes Weiner <hannes@cmpxchg.org>]

On Tue, Jun 23, 2020 at 11:45:14AM -0700, Roman Gushchin wrote:
> Memory cgroups are using large chunks of percpu memory to store vmstat
> data.  Yet this memory is not accounted at all, so in the case when there
> are many (dying) cgroups, it's not exactly clear where all the memory is.
> 
> Because the size of memory cgroup internal structures can dramatically
> exceed the size of object or page which is pinning it in the memory, it's
> not a good idea to simple ignore it.  It actually breaks the isolation
> between cgroups.
> 
> Let's account the consumed percpu memory to the parent cgroup.
> 
> Signed-off-by: Roman Gushchin <guro@fb.com>
> Acked-by: Dennis Zhou <dennis@kernel.org>

Acked-by: Johannes Weiner <hannes@cmpxchg.org>

This makes sense, and the accounting is in line with how we track and
distribute child creation quotas (cgroup.max.descendants and
cgroup.max.depth) up the cgroup tree.

I have one minor comment that isn't a dealbreaker for me:

> @@ -5069,13 +5069,15 @@ static int alloc_mem_cgroup_per_node_info(struct mem_cgroup *memcg, int node)
>  	if (!pn)
>  		return 1;
>  
> -	pn->lruvec_stat_local = alloc_percpu(struct lruvec_stat);
> +	pn->lruvec_stat_local = alloc_percpu_gfp(struct lruvec_stat,
> +						 GFP_KERNEL_ACCOUNT);
>  	if (!pn->lruvec_stat_local) {
>  		kfree(pn);
>  		return 1;
>  	}
>  
> -	pn->lruvec_stat_cpu = alloc_percpu(struct lruvec_stat);
> +	pn->lruvec_stat_cpu = alloc_percpu_gfp(struct lruvec_stat,
> +					       GFP_KERNEL_ACCOUNT);
>  	if (!pn->lruvec_stat_cpu) {
>  		free_percpu(pn->lruvec_stat_local);
>  		kfree(pn);
> @@ -5149,11 +5151,13 @@ static struct mem_cgroup *mem_cgroup_alloc(void)
>  		goto fail;
>  	}
>  
> -	memcg->vmstats_local = alloc_percpu(struct memcg_vmstats_percpu);
> +	memcg->vmstats_local = alloc_percpu_gfp(struct memcg_vmstats_percpu,
> +						GFP_KERNEL_ACCOUNT);
>  	if (!memcg->vmstats_local)
>  		goto fail;
>  
> -	memcg->vmstats_percpu = alloc_percpu(struct memcg_vmstats_percpu);
> +	memcg->vmstats_percpu = alloc_percpu_gfp(struct memcg_vmstats_percpu,
> +						 GFP_KERNEL_ACCOUNT);
>  	if (!memcg->vmstats_percpu)
>  		goto fail;
>  
> @@ -5202,7 +5206,9 @@ mem_cgroup_css_alloc(struct cgroup_subsys_state *parent_css)
>  	struct mem_cgroup *memcg;
>  	long error = -ENOMEM;
>  
> +	memalloc_use_memcg(parent);
>  	memcg = mem_cgroup_alloc();
> +	memalloc_unuse_memcg();

The disconnect between 1) requesting accounting and 2) which cgroup to
charge is making me uneasy. It makes mem_cgroup_alloc() a bit of a
handgrenade, because accounting to the current task is almost
guaranteed to be wrong if the use_memcg() annotation were to get lost
in a refactor or not make it to a new caller of the function.

The saving grace is that mem_cgroup_alloc() is pretty unlikely to be
used elsewhere. And pretending it's an independent interface would be
overengineering. But how about the following in mem_cgroup_alloc() and
alloc_mem_cgroup_per_node_info() to document that caller relationship:

	/* We charge the parent cgroup, never the current task */
	WARN_ON_ONCE(!current->active_memcg);