linux-kernel - Re: [PATCH -mm v9 1/8] memcg: add page_cgroup

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Date:	Tue, 21 Jul 2015 16:34:07 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Vladimir Davydov <vdavydov@...allels.com>
Cc:	Andres Lagar-Cavilla <andreslc@...gle.com>,
	Minchan Kim <minchan@...nel.org>,
	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.cz>,
	Greg Thelen <gthelen@...gle.com>,
	Michel Lespinasse <walken@...gle.com>,
	David Rientjes <rientjes@...gle.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	Jonathan Corbet <corbet@....net>, <linux-api@...r.kernel.org>,
	<linux-doc@...r.kernel.org>, <linux-mm@...ck.org>,
	<cgroups@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -mm v9 1/8] memcg: add page_cgroup_ino helper

On Sun, 19 Jul 2015 15:31:10 +0300 Vladimir Davydov <vdavydov@...allels.com> wrote:

> This function returns the inode number of the closest online ancestor of
> the memory cgroup a page is charged to. It is required for exporting
> information about which page is charged to which cgroup to userspace,
> which will be introduced by a following patch.
> 
> ...
>

> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -441,6 +441,29 @@ struct cgroup_subsys_state *mem_cgroup_css_from_page(struct page *page)
>  	return &memcg->css;
>  }
>  
> +/**
> + * page_cgroup_ino - return inode number of the memcg a page is charged to
> + * @page: the page
> + *
> + * Look up the closest online ancestor of the memory cgroup @page is charged to
> + * and return its inode number or 0 if @page is not charged to any cgroup. It
> + * is safe to call this function without holding a reference to @page.
> + */
> +unsigned long page_cgroup_ino(struct page *page)

Shouldn't it return an ino_t?

> +{
> +	struct mem_cgroup *memcg;
> +	unsigned long ino = 0;
> +
> +	rcu_read_lock();
> +	memcg = READ_ONCE(page->mem_cgroup);
> +	while (memcg && !(memcg->css.flags & CSS_ONLINE))
> +		memcg = parent_mem_cgroup(memcg);
> +	if (memcg)
> +		ino = cgroup_ino(memcg->css.cgroup);
> +	rcu_read_unlock();
> +	return ino;
> +}

The function is racy, isn't it?  There's nothing to prevent this inode
from getting torn down and potentially reallocated one nanosecond after
page_cgroup_ino() returns?  If so, it is only safely usable by things
which don't care (such as procfs interfaces) and this should be
documented in some fashion.
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