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Date: Tue, 13 Mar 2018 14:49:02 +0100
From: Michal Hocko <mhocko@kernel.org>
To: Shakeel Butt <shakeelb@google.com>
Cc: Jan Kara <jack@suse.cz>, Amir Goldstein <amir73il@gmail.com>,
        Christoph Lameter <cl@linux.com>, Pekka Enberg <penberg@kernel.org>,
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Subject: Re: [PATCH v3 1/2] mm: memcg: remote memcg charging for kmem
 allocations
Message-ID: <20180313134902.GW12772@dhcp22.suse.cz>
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 <20180221223757.127213-2-shakeelb@google.com>
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On Wed 21-02-18 14:37:56, Shakeel Butt wrote:
[...]
> +#ifdef CONFIG_MEMCG
> +static inline struct mem_cgroup *memalloc_memcg_save(struct mem_cgroup *memcg)
> +{
> +	struct mem_cgroup *old_memcg = current->target_memcg;
> +	current->target_memcg = memcg;
> +	return old_memcg;
> +}

So you are relying that the caller will handle the reference counting
properly? I do not think this is a good idea. Also do we need some kind
of debugging facility to detect unbalanced save/restore scopes?

[...]
> @@ -2260,7 +2269,10 @@ struct kmem_cache *memcg_kmem_get_cache(struct kmem_cache *cachep)
>  	if (current->memcg_kmem_skip_account)
>  		return cachep;
>  
> -	memcg = get_mem_cgroup_from_mm(current->mm);
> +	if (current->target_memcg)
> +		memcg = get_mem_cgroup(current->target_memcg);
> +	if (!memcg)
> +		memcg = get_mem_cgroup_from_mm(current->mm);
>  	kmemcg_id = READ_ONCE(memcg->kmemcg_id);
>  	if (kmemcg_id < 0)
>  		goto out;

You are also adding one branch for _each_ charge path even though the
usecase is rather limited.

I will have to think about this approach more. It is clearly less code
than your previous attempt but I cannot say I would be really impressed.
-- 
Michal Hocko
SUSE Labs