Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Thu, 7 Nov 2019 07:44:13 -0800
From:   Tejun Heo <tj@kernel.org>
To:     Roman Gushchin <guro@fb.com>
Cc:     linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
        Michal Hocko <mhocko@kernel.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        linux-kernel@vger.kernel.org, kernel-team@fb.com,
        stable@vger.kernel.org
Subject: Re: [PATCH 2/2] mm: hugetlb: switch to css_tryget() in
 hugetlb_cgroup_charge_cgroup()
Message-ID: <20191107154413.GZ3622521@devbig004.ftw2.facebook.com>
References: <20191106225131.3543616-1-guro@fb.com>
 <20191106225131.3543616-2-guro@fb.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20191106225131.3543616-2-guro@fb.com>
User-Agent: Mutt/1.5.21 (2010-09-15)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Wed, Nov 06, 2019 at 02:51:31PM -0800, Roman Gushchin 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>
> Cc: stable@vger.kernel.org
> Cc: Tejun Heo <tj@kernel.org>

Acked-by: Tejun Heo <tj@kernel.org>

Thanks.

-- 
tejun