Date: Fri, 22 May 2015 19:28:31 -0400
From: Johannes Weiner <hannes@cmpxchg.org>
To: Tejun Heo <tj@kernel.org>
Cc: axboe@kernel.dk, linux-kernel@vger.kernel.org, jack@suse.cz,
        hch@infradead.org, linux-fsdevel@vger.kernel.org, vgoyal@redhat.com,
        lizefan@huawei.com, cgroups@vger.kernel.org, linux-mm@kvack.org,
        mhocko@suse.cz, clm@fb.com, fengguang.wu@intel.com,
        david@fromorbit.com, gthelen@google.com, khlebnikov@yandex-team.ru
Subject: Re: [PATCH 11/51] memcg: implement mem_cgroup_css_from_page()
Message-ID: <20150522232831.GB6485@cmpxchg.org>
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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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