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Subject: Re: [PATCH] mm/memcg: fix device private memcg accounting
To:     Andrew Morton <akpm@linux-foundation.org>
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References: <20201009215952.2726-1-rcampbell@nvidia.com>
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From:   Ralph Campbell <rcampbell@nvidia.com>
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On 10/9/20 3:50 PM, Andrew Morton wrote:
> 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?
> 

I assume the memory cgroup accounting would be off somehow when moving
a process to another memory cgroup.
Currently, the device private page is charged like a normal anonymous page
when allocated and is uncharged when the page is freed so I think that path is OK.
Maybe someone who knows more about memory cgroup accounting can comment?