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

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Message-ID: <CAPM=9tzkLXOz=-3eujUbbjMHunR+_5JZ4oQaqNmbrWWF9WZJ0w@mail.gmail.com>
Date: Tue, 1 Apr 2025 06:43:43 +1000
From: Dave Airlie <airlied@...il.com>
To: Maxime Ripard <mripard@...nel.org>
Cc: Christian König <christian.koenig@....com>, 
	Andrew Morton <akpm@...ux-foundation.org>, Marek Szyprowski <m.szyprowski@...sung.com>, 
	Robin Murphy <robin.murphy@....com>, Sumit Semwal <sumit.semwal@...aro.org>, 
	Benjamin Gaignard <benjamin.gaignard@...labora.com>, Brian Starkey <Brian.Starkey@....com>, 
	John Stultz <jstultz@...gle.com>, "T.J. Mercier" <tjmercier@...gle.com>, 
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, Thomas Zimmermann <tzimmermann@...e.de>, 
	Simona Vetter <simona@...ll.ch>, Tomasz Figa <tfiga@...omium.org>, 
	Mauro Carvalho Chehab <mchehab@...nel.org>, Ben Woodard <woodard@...hat.com>, 
	Hans Verkuil <hverkuil@...all.nl>, 
	Laurent Pinchart <laurent.pinchart+renesas@...asonboard.com>, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, iommu@...ts.linux.dev, 
	linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org, 
	linaro-mm-sig@...ts.linaro.org
Subject: Re: [PATCH RFC 00/12] dma: Enable dmem cgroup tracking

On Tue, 11 Mar 2025 at 00:26, Maxime Ripard <mripard@...nel.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