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Date:   Wed, 6 Nov 2019 16:26:44 -0800
From:   Johannes Weiner <hannes@cmpxchg.org>
To:     Roman Gushchin <guro@fb.com>
Cc:     linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
        Michal Hocko <mhocko@kernel.org>, linux-kernel@vger.kernel.org,
        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()
Message-ID: <20191107002644.GB96548@cmpxchg.org>
References: <20191106225131.3543616-1-guro@fb.com>
 <20191106225131.3543616-2-guro@fb.com>
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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>

Acked-by: Johannes Weiner <hannes@cmpxchg.org>