Re: Pinned, non-revocable mappings of VRAM: will bad things happen?

[Demi Marie Obenour <demiobenour@gmail.com>]

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On 4/16/26 05:57, Christian König wrote:
> On 4/16/26 01:27, Demi Marie Obenour wrote:
>> Is it safe to assume that if a dmabuf exporter cannot handle
>> non-revocable, pinned importers, it will fail the import?  Or is
>> using dma_buf_pin() unsafe if one does not know the exporter?
> 
> Neither.
> 
> dma_buf_pin() makes sure that the importer doesn't get any invalidation notifications because the exporter moves the backing store of the buffer around for memory management.
> 
> But what is still possible is that the exporter is hot removed, in which case the importer should basically terminate it's DMA operation as soon as possible.
> 
> GPU drivers usually reject pin requests to VRAM from DMA-buf importers when that isn't restricted by cgroups for example, because that can otherwise easily result in a deny of service.
> 
> Amdgpu only recently started to allow pinning into VRAM to support RDMA without ODP (I think it was ODP, but could be that I mixed up the RDMA three letter code for that feature).
> 
>> For context, Xen grant tables do not support revocation.  One can ask
>> the guest to unmap the grants, but if the guest doesn't obey the only
>> recourse is to ungracefully kill it.  They also do not support page
>> faults, so the pages must be pinned.  Right now, grant tables don't
>> support PCI BAR mappings, but that's fixable.
> 
> That sounds like an use case for the DMA-buf pin interface.
> 
>> How badly is this going to break with dGPU VRAM, if at all?  I know
>> that AMDGPU has a fallback when the BAR isn't mappable.  What about
>> other drivers?  Supporting page faults the way KVM does is going to
>> be extremely hard, so pinned mappings and DMA transfers are vastly
>> preferable.
> 
> Well if you only want to share a fixed amount of VRAM then that is pretty much ok.
> 
> But when the client VM can trigger pinning on demand without any limitation you can pretty easily have deny of service against the host. That is usually a rather bad idea.

Is there a reasonable way to choose such an amount?  Unless I am
mistaken, client workloads are highly non-uniform: a single game or
compute job might well use more VRAM than every other program on the
system combined.  Are these workloads impossible to make work well
with pinning?
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)

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