Re: [PATCH v2 0/3] dma-buf: heaps: cma: enable dmem cgroup accounting

[Maxime Ripard]

Hi TJ,

On Thu, Feb 19, 2026 at 05:14:42PM -0800, T.J. Mercier wrote:
> On Wed, Feb 18, 2026 at 9:15 AM Eric Chanudet <echanude@xxxxxxxxxx> wrote:
> > An earlier series[1] from Maxime introduced dmem to the cma allocator in
> > an attempt to use it generally for dma-buf. Restart from there and apply
> > the charge in the narrower context of the CMA dma-buf heap instead.
> >
> > In line with introducing cgroup to the system heap[2], this behavior is
> > enabled based on dma_heap.mem_accounting, disabled by default.
> >
> > dmem is chosen for CMA heaps as it allows limits to be set for each
> > region backing each heap. The charge is only put in the dma-buf heap for
> > now as it guaranties it can be accounted against a userspace process
> > that requested the allocation.
> 
> But CMA memory is system memory, and regular (non-CMA) movable
> allocations can occur out of these CMA areas. So this splits system
> memory accounting between memcg (from [2]) and dmem. If I want to put
> a limit on system memory use I have to adjust multiple limits (memcg +
> dmems) and know how to divide the total between them all.
> 
> How do you envision using this combination of different controllers?

I feel like it can be argued either way, and I don't really see a way
out of supporting both.

Like you pointed out, CMA can indeed be seen as system memory, but it's
also a limited pool that you might want to place arbitrary limits on
since, unlike system memory, it can't be reclaimed, will not trigger the
OOM killer, and more generally is a much more sparse resource.

Maxime

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