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

["Christian König" <christian.koenig@amd.com>]

On 2/24/26 10:43, Maxime Ripard wrote:
> Hi Christian,
> 
> On Fri, Feb 20, 2026 at 10:45:08AM +0100, Christian König wrote:
>> On 2/20/26 02:14, T.J. Mercier wrote:
>>> On Wed, Feb 18, 2026 at 9:15 AM Eric Chanudet <echanude@redhat.com> wrote:
>>>
>>> Hi Eric,
>>>
>>>> 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?
>>
>> Yeah we have this problem pretty much everywhere.
>>
>> There are both use cases where you want to account device allocations
>> to memcg and when you don't want that.
>>
>> From what I know at the moment it would be best if the administrator
>> could say for each dmem if it should account additionally to memcg or
>> not.
>>
>> Using module parameters to enable/disable it globally is just a
>> workaround as far as I can see.
> 
> That's a pretty good idea! It would indeed be a solution that could
> satisfy everyone (I assume?).

I think so yeah.

From what I have seen we have three different use cases:

1. local device memory (VRAM), GTT/CMA and memcg are completely separate domains and you want to have completely separate values as limit for them.

2. local device memory (VRAM) is separate. GTT/CMA are accounted to memcg, you can still have separate values as limit so that nobody over allocates CMA (for example).

3. All three are accounted to memcg because system memory is actually used as fallback if applications over allocate device local memory.

It's debatable what should be the default, but we clearly need to handle all three use cases. Potentially even on the same system.

Regards,
Christian.

> 
> Maxime