Re: [PATCH RFC 00/12] dma: Enable dmem cgroup tracking

Dave Airlie
2025-03-31T20:43:56Z

On Tue, 11 Mar 2025 at 00:26, Maxime Ripard <mripard@kernel.org> wrote:
>
> Hi,
>
> On Mon, Mar 10, 2025 at 03:16:53PM +0100, Christian König wrote:
> > [Adding Ben since we are currently in the middle of a discussion
> > regarding exactly that problem]
> >
> > Just for my understanding before I deep dive into the code: This uses
> > a separate dmem cgroup and does not account against memcg, don't it?
>
> Yes. The main rationale being that it doesn't always make sense to
> register against memcg: a lot of devices are going to allocate from
> dedicated chunks of memory that are either carved out from the main
> memory allocator, or not under Linux supervision at all.
>
> And if there's no way to make it consistent across drivers, it's not the
> right tool.
>

While I agree on that, if a user can cause a device driver to allocate
memory that is also memory that memcg accounts, then we have to
interface with memcg to account that memory.

The pathological case would be a single application wanting to use 90%
of RAM for device allocations, freeing it all, then using 90% of RAM
for normal usage. How to create a policy that would allow that with
dmem and memcg is difficult, since if you say you can do 90% on both
then the user can easily OOM the system.

Dave.
> Maxime