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

[Barry Song]

On Mon, May 18, 2026 at 3:34 PM Christian König
<christian.koenig at amd.com> wrote:
>
> On 5/16/26 11:19, Barry Song wrote:
> > On Thu, May 14, 2026 at 12:35 AM T.J. Mercier <tjmercier at 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.
>

Hi Christian,

Thanks very much for your explanation. So basically it seems that
dma_buf_export() is not the proper place to charge, since it may end up
mixing in non-system-memory accounting?

My question is also about the global view for both heap and non-heap cases.
After reading the discussion, I’ve tried to summarize it—please let me know
if my understanding is correct.

for dma_heap, we have the ioctl DMA_HEAP_IOCTL_ALLOC, where users can pass a
remote pidfd or similar information to indicate where the dma-buf should be
charged, as in Albert's patchset.

For non-dma_heap dma-bufs, we don’t have an obvious userspace entry point that
triggers the allocation. So we likely need other approaches. We could either
move more drivers over to dma-heap, or introduce something like
DMA_BUF_IOCTL_XFER_CHARGE, as you are discussing, to let userspace explicitly
declare a charge.

Best Regards
Barry