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Date:   Fri, 9 Oct 2020 15:50:55 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Ralph Campbell <rcampbell@nvidia.com>
Cc:     <linux-mm@kvack.org>, <cgroups@vger.kernel.org>,
        <linux-kernel@vger.kernel.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@kernel.org>,
        Vladimir Davydov <vdavydov.dev@gmail.com>,
        =?ISO-8859-1?Q?J=E9r=F4me?= Glisse <jglisse@redhat.com>,
        Balbir Singh <bsingharora@gmail.com>,
        Ira Weiny <ira.weiny@intel.com>, <stable@vger.kernel.org>
Subject: Re: [PATCH] mm/memcg: fix device private memcg accounting
Message-Id: <20201009155055.f87de51ea04d4ea879e3981a@linux-foundation.org>
In-Reply-To: <20201009215952.2726-1-rcampbell@nvidia.com>
References: <20201009215952.2726-1-rcampbell@nvidia.com>
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On Fri, 9 Oct 2020 14:59:52 -0700 Ralph Campbell <rcampbell@nvidia.com> wrote:

> The code in mc_handle_swap_pte() checks for non_swap_entry() and returns
> NULL before checking is_device_private_entry() so device private pages
> are never handled.
> Fix this by checking for non_swap_entry() after handling device private
> swap PTEs.
> 
> Cc: stable@vger.kernel.org

I was going to ask "what are the end-user visible effects of the bug". 
This is important information with a cc:stable.

> 
> I'm not sure exactly how to test this. I ran the HMM self tests but
> that is a minimal sanity check. I think moving the self test from one
> memory cgroup to another while it is running would exercise this patch.
> I'm looking at how the test could move itself to another group after
> migrating some anonymous memory to the test driver.
> 

But this makes me suspect the answer is "there aren't any that we know
of".  Are you sure a cc:stable is warranted?