linux-kernel - Re: [PATCH 11/51] memcg: implement mem_cgroup_css_from

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Date:	Fri, 22 May 2015 19:28:31 -0400
From:	Johannes Weiner <hannes@...xchg.org>
To:	Tejun Heo <tj@...nel.org>
Cc:	axboe@...nel.dk, linux-kernel@...r.kernel.org, jack@...e.cz,
	hch@...radead.org, linux-fsdevel@...r.kernel.org,
	vgoyal@...hat.com, lizefan@...wei.com, cgroups@...r.kernel.org,
	linux-mm@...ck.org, mhocko@...e.cz, clm@...com,
	fengguang.wu@...el.com, david@...morbit.com, gthelen@...gle.com,
	khlebnikov@...dex-team.ru
Subject: Re: [PATCH 11/51] memcg: implement mem_cgroup_css_from_page()

On Fri, May 22, 2015 at 05:13:25PM -0400, Tejun Heo wrote:
> +/**
> + * mem_cgroup_css_from_page - css of the memcg associated with a page
> + * @page: page of interest
> + *
> + * This function is guaranteed to return a valid cgroup_subsys_state and
> + * the returned css remains accessible until @page is released.
> + */
> +struct cgroup_subsys_state *mem_cgroup_css_from_page(struct page *page)
> +{
> +	if (page->mem_cgroup)
> +		return &page->mem_cgroup->css;
> +	return &root_mem_cgroup->css;
> +}

replace_page_cache() can clear page->mem_cgroup even when the page
still has references, so unfortunately you must hold the page lock
when calling this function.

I haven't checked how you use this - chances are you always have the
page locked anyways - but it probably needs a comment.
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