linux-kernel - Re: [PATCH v1 3/4] mm: introduce page memcg flags

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Message-ID: <20200924200542.GD329853@cmpxchg.org>
Date:   Thu, 24 Sep 2020 16:05:42 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Roman Gushchin <guro@...com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Michal Hocko <mhocko@...nel.org>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, kernel-team@...com
Subject: Re: [PATCH v1 3/4] mm: introduce page memcg flags

On Tue, Sep 22, 2020 at 01:36:59PM -0700, Roman Gushchin wrote:
> @@ -422,7 +421,13 @@ static inline void clear_page_mem_cgroup(struct page *page)
>   */
>  static inline struct obj_cgroup **page_obj_cgroups(struct page *page)
>  {
> -	return (struct obj_cgroup **)(page->memcg_data & ~0x1UL);
> +	unsigned long memcg_data = page->memcg_data;
> +
> +	VM_BUG_ON_PAGE(memcg_data && !test_bit(PG_MEMCG_OBJ_CGROUPS,
> +					       &memcg_data), page);
> +	__clear_bit(PG_MEMCG_OBJ_CGROUPS, &memcg_data);
> +
> +	return (struct obj_cgroup **)memcg_data;

Slab allocations set up page->memcg_data locklessly, right? AFAICS,
the page_objcg lookup functions all need READ_ONCE() loads.