Re: [Linaro-mm-sig] Re: [PATCH RFC 2/5] dma-heap: charge dma-buf memory via explicit memcg

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

On 5/16/26 11:19, Barry Song wrote:
> On Thu, May 14, 2026 at 12:35 AM T.J. Mercier <tjmercier@google.com> wrote:
> [...]
>>>> I have a question about this part. Albert I guess you are interested
>>>> only in accounting dmabuf-heap allocations, or do you expect to add
>>>> __GFP_ACCOUNT or mem_cgroup_charge_dmabuf calls to other
>>>> non-dmabuf-heap exporters?
>>>
>>> We're scoping this to dma-buf heaps for now. CMA heaps and the dmem
>>> controller are on the radar for follow-up/parallel work (there will be
>>> dragons and will surely need discussion). For DRM and V4L2 the
>>> long-term intent is migration to heaps, which would make direct
>>> accounting on those paths unnecessary.
>>
>> Ah I see. GEM buffers exported to dmabufs are what I had in mind. I
>> guess this would only leave the odd non-DRM driver with the need to
>> add their own accounting calls, which I don't expect would be a big
>> problem.
>>
> 
> sounds like we still have a long way to go to correctly account for
> various v4l2, drm, GEM, CMA, etc. In patch 1, the charging is done in
> dma_buf_export(), so I guess it covers all dma-buf types except
> dma_heap, but the problem is that it has no remote charging support at
> all?

No, just the other way around

DMA-buf heaps can be handled here because we know that it is pure system memory and nothing special so memcg always applies.

dma_buf_export() on the other hand handles tons of different use cases, ranging from buffer accounted to dmem, over special resources which aren't even memory all the way to buffers which can migrate from dmem to memcg and back during their lifetime.

>>> udmabufs are already
>>> memcg-charged, so adding a separate MEMCG_DMABUF would double count.
>>> Are there any other exporters you had in mind that would benefit from
>>> this approach?

Well apart from DMA-buf memfd_create() is one of the things which as broken our neck in the past a couple of times.

But thinking more about it what if instead of making this DMA-buf heaps specific what if we have a general cgroups function which allows to change accounting of a buffer referenced by a file descriptor to a different process?

That would cover not only the DMA-buf heaps use case, but also all other DMA-buf with dmem and whatever we come up in the future as well.

The only drawback I can see is that DMA-buf heap allocations would be temporarily accounted to the memory allocation daemon, but I don't think that this would be a problem.

Regards,
Christian.

> 
> Thanks
> Barry