Re: [PATCH] dma-buf: system_heap: account for system heap allocation in memcg

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

On 12/19/25 16:58, Maxime Ripard wrote:
> On Fri, Dec 19, 2025 at 02:50:50PM +0100, Christian König wrote:
>> On 12/19/25 11:25, Maxime Ripard wrote:
>>> On Mon, Dec 15, 2025 at 03:53:22PM +0100, Christian König wrote:
>>>> On 12/15/25 14:59, Maxime Ripard wrote:
>> ...
>>>>>>> The shared ownership is indeed broken, but it's not more or less broken
>>>>>>> than, say, memfd + udmabuf, and I'm sure plenty of others.
>>>>>>>
>>>>>>> So we really improve the common case, but only make the "advanced"
>>>>>>> slightly more broken than it already is.
>>>>>>>
>>>>>>> Would you disagree?
>>>>>>
>>>>>> I strongly disagree. As far as I can see there is a huge chance we
>>>>>> break existing use cases with that.
>>>>>
>>>>> Which ones? And what about the ones that are already broken?
>>>>
>>>> Well everybody that expects that driver resources are *not* accounted to memcg.
>>>
>>> Which is a thing only because these buffers have never been accounted
>>> for in the first place.
>>
>> Yeah, completely agree. By not accounting it for such a long time we
>> ended up with people depending on this behavior.
>>
>> Not nice, but that's what it is.
>>
>>> So I guess the conclusion is that we shouldn't
>>> even try to do memory accounting, because someone somewhere might not
>>> expect that one of its application would take too much RAM in the
>>> system?
>>
>> Well we do need some kind of solution to the problem. Either having
>> some setting where you say "This memcg limit is inclusive/exclusive
>> device driver allocated memory" or have a completely separate limit
>> for device driver allocated memory.
> 
> A device driver memory specific limit sounds like a good idea because it
> would make it easier to bridge the gap with dmem.

Completely agree, but that approach was rejected by the cgroups people.

I mean we can already use udmabuf to allocate memcg accounted system memory which then can be imported into device drivers.

So I don't see much reason why we should account dma-buf heaps and driver interfaces to memcg as well, we just need some way to limit them.

Regards,
Christian.

> 
> Happy holidays,
> Maxime