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

[Ralph Campbell]

On 10/9/20 3:50 PM, Andrew Morton wrote:
> On Fri, 9 Oct 2020 14:59:52 -0700 Ralph Campbell <rcampbell@xxxxxxxxxx> 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@xxxxxxxxxxxxxxx
>
> 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?