linux-kernel - Re: [PATCH v2 07/28] mm: memcg/slab: introduce mem_cgroup_from

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Date:   Mon, 3 Feb 2020 11:05:06 -0500
From:   Johannes Weiner <hannes@...xchg.org>
To:     Roman Gushchin <guro@...com>
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...nel.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        linux-kernel@...r.kernel.org, kernel-team@...com,
        Bharata B Rao <bharata@...ux.ibm.com>,
        Yafang Shao <laoar.shao@...il.com>
Subject: Re: [PATCH v2 07/28] mm: memcg/slab: introduce mem_cgroup_from_obj()

On Mon, Jan 27, 2020 at 09:34:32AM -0800, Roman Gushchin wrote:
> @@ -757,13 +757,12 @@ void __mod_lruvec_state(struct lruvec *lruvec, enum node_stat_item idx,
>  
>  void __mod_lruvec_slab_state(void *p, enum node_stat_item idx, int val)
>  {
> -	struct page *page = virt_to_head_page(p);
> -	pg_data_t *pgdat = page_pgdat(page);
> +	pg_data_t *pgdat = page_pgdat(virt_to_page(p));
>  	struct mem_cgroup *memcg;
>  	struct lruvec *lruvec;
>  
>  	rcu_read_lock();
> -	memcg = memcg_from_slab_page(page);
> +	memcg = mem_cgroup_from_obj(p);
>  
>  	/* Untracked pages have no memcg, no lruvec. Update only the node */
>  	if (!memcg || memcg == root_mem_cgroup) {

This function is specifically for slab objects. Why does it need the
indirection and additional branch here?

If memcg_from_slab_page() is going away later, I think the conversion
to this new helper should happen at that point in the series, not now.