On Tue, May 19, 2026 at 12:10 AM Christian König
<[email protected]> wrote:
>
> On 5/19/26 01:00, Barry Song wrote:
> > On Mon, May 18, 2026 at 3:34 PM Christian König
> > <[email protected]> wrote:
> >>
> >> On 5/16/26 11:19, Barry Song wrote:
> >>> On Thu, May 14, 2026 at 12:35 AM T.J. Mercier <[email protected]>
> >>> 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?
>
> Yes, exactly that.
>
> > 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.
>
> Well that's the current proposal, but I think we need to come up with
> something more general.
>
> > 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.
>
> Yeah but that's not only for DMA-buf, we need that for file descriptors
> returned by memfd_create() as well.
memfds get charged on fault, so an allocator shouldn't currently be
charged just for creating the fd. Unlike system/CMA heap buffers, the
shmem backing a memfd / udmabuf is LRU memory, and swapping the memcg
owner of those pages is a more-involved process which is not supported
by memcg v2. There used to be some support in memcg v1, but it was
removed. Commit e548ad4a7cbf ("mm: memcg: move charge migration code
to memcontrol-v1.c ") said, "It's a fairly large and complicated code
which created a number of problems in the past." So I'm not sure how
much appetite there would be to support it in v2 for this.
