linux-kernel - Re: [PATCH v4 4/4] mm: convert page kmemcg type to a page memcg flag

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Message-ID: <20201001170036.GA29848@blackbook>
Date:   Thu, 1 Oct 2020 19:00:36 +0200
From:   Michal Koutný <mkoutny@...e.com>
To:     Roman Gushchin <guro@...com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, kernel-team@...com
Subject: Re: [PATCH v4 4/4] mm: convert page kmemcg type to a page memcg flag

Hi.

On Wed, Sep 30, 2020 at 05:27:10PM -0700, Roman Gushchin <guro@...com> wrote:
> @@ -369,8 +371,12 @@ enum page_memcg_data_flags {
>   */
>  static inline struct mem_cgroup *page_memcg(struct page *page)
>  {
> +	unsigned long memcg_data = page->memcg_data;
> +
>  	VM_BUG_ON_PAGE(PageSlab(page), page);
> -	return (struct mem_cgroup *)page->memcg_data;
> +	VM_BUG_ON_PAGE(memcg_data & MEMCG_DATA_OBJCGS, page);
> +
> +	return (struct mem_cgroup *)(memcg_data & ~MEMCG_DATA_FLAGS_MASK);
>  }
Shouldn't this change go also into page_memcg_rcu()? (I don't think the
current single user (workingset_activation() would pass a non-slab
kernel page but for consistency sake.)

Alternatively, I'm thinking why (in its single use) is there
page_memcg_rcu() a separate function to page_memcg() (cross memcg page
migration?).

Regards,
Michal

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