linux-kernel - Re: [PATCH] mm/memcg: fix device private memcg accounting

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Date:   Mon, 12 Oct 2020 10:11:07 -0700
From:   Ralph Campbell <rcampbell@...dia.com>
To:     Johannes Weiner <hannes@...xchg.org>
CC:     Andrew Morton <akpm@...ux-foundation.org>, <linux-mm@...ck.org>,
        <cgroups@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        Michal Hocko <mhocko@...nel.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Balbir Singh <bsingharora@...il.com>,
        Ira Weiny <ira.weiny@...el.com>, <stable@...r.kernel.org>
Subject: Re: [PATCH] mm/memcg: fix device private memcg accounting


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@...dia.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@...xchg.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.