Re: [PATCH 3/7] lru: add an element to a memcg list

[Greg Thelen <gthelen@google.com>]

On Fri, Feb 08 2013, Glauber Costa wrote:

> With the infrastructure we now have, we can add an element to a memcg
> LRU list instead of the global list. The memcg lists are still
> per-node.

[...]

> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index b9e1941..bfb4b5b 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -3319,6 +3319,36 @@ static inline void memcg_resume_kmem_account(void)
>  	current->memcg_kmem_skip_account--;
>  }
>  
> +static struct mem_cgroup *mem_cgroup_from_kmem_page(struct page *page)
> +{
> +	struct page_cgroup *pc;
> +	struct mem_cgroup *memcg = NULL;
> +
> +	pc = lookup_page_cgroup(page);
> +	if (!PageCgroupUsed(pc))
> +		return NULL;
> +
> +	lock_page_cgroup(pc);
> +	if (PageCgroupUsed(pc))
> +		memcg = pc->mem_cgroup;
> +	unlock_page_cgroup(pc);

Once we drop the lock, is there anything that needs protection
(e.g. PageCgroupUsed)?  If there's no problem, then what's the point of
taking the lock?

> +	return memcg;
> +}
> +

[...]

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