Re: [PATCH 2/2] mm: hugetlb: switch to css_tryget() in hugetlb_cgroup_charge_cgroup()

From: Shakeel Butt <shakeelb@google.com>
To: Roman Gushchin <guro@fb.com>
Cc: Linux MM <linux-mm@kvack.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Michal Hocko <mhocko@kernel.org>,
	Johannes Weiner <hannes@cmpxchg.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Kernel Team <kernel-team@fb.com>,
	stable@vger.kernel.org, Tejun Heo <tj@kernel.org>
Subject: Re: [PATCH 2/2] mm: hugetlb: switch to css_tryget() in hugetlb_cgroup_charge_cgroup()
Date: Wed, 6 Nov 2019 18:31:56 -0800	[thread overview]
Message-ID: <CALvZod7ickiX_D=7cJ_q4qjXebr93B7kh+WU61dagskA3Ye+0Q@mail.gmail.com> (raw)
In-Reply-To: <20191106225131.3543616-2-guro@fb.com>

On Wed, Nov 6, 2019 at 2:53 PM Roman Gushchin <guro@fb.com> wrote:
>
> An exiting task might belong to an offline cgroup. In this case
> an attempt to grab a cgroup reference from the task can end up
> with an infinite loop in hugetlb_cgroup_charge_cgroup(), because
> neither the cgroup will become online, neither the task will
> be migrated to a live cgroup.
>
> Fix this by switching over to css_tryget(). As css_tryget_online()
> can't guarantee that the cgroup won't go offline, in most cases
> the check doesn't make sense. In this particular case users of
> hugetlb_cgroup_charge_cgroup() are not affected by this change.
>
> A similar problem is described by commit 18fa84a2db0e ("cgroup: Use
> css_tryget() instead of css_tryget_online() in task_get_css()").
>
> Signed-off-by: Roman Gushchin <guro@fb.com>

Reviewed-by: Shakeel Butt <shakeelb@google.com>

> Cc: stable@vger.kernel.org
> Cc: Tejun Heo <tj@kernel.org>
> ---
>  mm/hugetlb_cgroup.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/hugetlb_cgroup.c b/mm/hugetlb_cgroup.c
> index f1930fa0b445..2ac38bdc18a1 100644
> --- a/mm/hugetlb_cgroup.c
> +++ b/mm/hugetlb_cgroup.c
> @@ -196,7 +196,7 @@ int hugetlb_cgroup_charge_cgroup(int idx, unsigned long nr_pages,
>  again:
>         rcu_read_lock();
>         h_cg = hugetlb_cgroup_from_task(current);
> -       if (!css_tryget_online(&h_cg->css)) {
> +       if (!css_tryget(&h_cg->css)) {
>                 rcu_read_unlock();
>                 goto again;
>         }
> --
> 2.17.1
>