Re: [PATCH 2/2] mm: remove device private page support from hmm_range_fault

[Ralph Campbell <rcampbell@nvidia.com>]

On 3/16/20 1:09 PM, Jason Gunthorpe wrote:
> On Mon, Mar 16, 2020 at 07:49:35PM +0100, Christoph Hellwig wrote:
>> On Mon, Mar 16, 2020 at 11:42:19AM -0700, Ralph Campbell wrote:
>>>
>>> On 3/16/20 10:52 AM, Christoph Hellwig wrote:
>>>> No driver has actually used properly wire up and support this feature.
>>>> There is various code related to it in nouveau, but as far as I can tell
>>>> it never actually got turned on, and the only changes since the initial
>>>> commit are global cleanups.
>>>
>>> This is not actually true. OpenCL 2.x does support SVM with nouveau and
>>> device private memory via clEnqueueSVMMigrateMem().
>>> Also, Ben Skeggs has accepted a set of patches to map GPU memory after being
>>> migrated and this change would conflict with that.
>>
>> Can you explain me how we actually invoke this code?
>>
>> For that we'd need HMM_PFN_DEVICE_PRIVATE NVIF_VMM_PFNMAP_V0_VRAM
>> set in ->pfns before calling hmm_range_fault, which isn't happening.
> 
> Oh, I got tripped on this too
> 
> The logic is backwards from what you'd think.. If you *set*
> HMM_PFN_DEVICE_PRIVATE then this triggers:
> 
> hmm_pte_need_fault():
> 	if ((cpu_flags & range->flags[HMM_PFN_DEVICE_PRIVATE])) {
> 		/* Do we fault on device memory ? */
> 		if (pfns & range->flags[HMM_PFN_DEVICE_PRIVATE]) {
> 			*write_fault = pfns & range->flags[HMM_PFN_WRITE];
> 			*fault = true;
> 		}
> 		return;
> 	}
> 
> Ie if the cpu page is a DEVICE_PRIVATE and the caller sets
> HMM_PFN_DEVICE_PRIVATE in the input flags (pfns) then it always faults
> it and never sets HMM_PFN_DEVICE_PRIVATE in the output flags.
> 
> So setting 0 enabled device_private support, and nouveau is Ok.
> 
> AMDGPU is broken because it can't handle device private and can't set
> the flag to supress it.
> 
> I was going to try and sort this out as part of getting rid of range->flags
> 
> Jason
> 

The reason for it being backwards is that "normally" a device doesn't want
the device private page to be faulted back to system memory, it wants to
get the device private struct page so it can update its page table to point
to the memory already in the device.
Also, a device like Nvidia's GPUs may have an alternate path for copying
one GPU's memory to another (nvlink) without going through system memory
so getting a device private struct page/PFN from hmm_range_fault() that isn't
"owned" by the faulting GPU is useful.
I agree that the current code isn't well tested or thought out for multiple devices
(rdma, NVMe drives, GPUs, etc.) but it also ties in with peer-to-peer access via PCIe.
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