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SubjectRe: [PATCH] mm/memcg: fix device private memcg accounting
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On 10/12/20 6:28 AM, Johannes Weiner wrote:
> On Fri, Oct 09, 2020 at 05:00:37PM -0700, Ralph Campbell wrote:
>>
>> 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.
>
> The fix looks good to me.
>
> Acked-by: Johannes Weiner <hannes@cmpxchg.org>
>
>>> 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?
>
> As for whether to CC stable, I'm leaning toward no:
>
> - When moving tasks, we'd leave their device pages behind in the old
> cgroup. This isn't great, but it doesn't cause counter imbalances or
> corruption or anything - we also skip locked pages, we used to skip
> pages mapped by more than one pte, the user can select whether to
> move pages along tasks at all, and if so, whether only anon or file.
>
> - Charge moving itself is a bit of a questionable feature, and users
> have been moving away from it. Leaving tasks in a cgroup and
> changing the configuration is a heck of a lot cheaper than moving
> potentially gigabytes of pages to another configuration domain.
>
> - According to the Fixes tag, this isn't a regression, either. Since
> their inception, we have never migrated device pages.

Thanks for the Acked-by and the comments.
I assume Andrew will update the tags when queuing this up unless he wants me to resend.

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