Re: [PATCH v2 1/5] mm/hmm: HMM API to enable P2P DMA for device private pages

[Yonatan Maman <ymaman@nvidia.com>]

On 21/07/2025 9:59, Christoph Hellwig wrote:
> On Fri, Jul 18, 2025 at 02:51:08PM +0300, Yonatan Maman wrote:
>> From: Yonatan Maman <Ymaman@Nvidia.com>
>>
>> hmm_range_fault() by default triggered a page fault on device private
>> when HMM_PFN_REQ_FAULT flag was set. pages, migrating them to RAM. In some
>> cases, such as with RDMA devices, the migration overhead between the
>> device (e.g., GPU) and the CPU, and vice-versa, significantly degrades
>> performance. Thus, enabling Peer-to-Peer (P2P) DMA access for device
>> private page might be crucial for minimizing data transfer overhead.
> 
> You don't enable DMA for device private pages.  You allow discovering
> a DMAable alias for device private pages.
>
> Also absolutely nothing GPU specific here.
>
Ok, understood, I will change it (v3).
  >> +	/*
>> +	 * Don't fault in device private pages owned by the caller,
>> +	 * just report the PFN.
>> +	 */
>> +	if (pgmap->owner == range->dev_private_owner) {
>> +		*hmm_pfn = swp_offset_pfn(entry);
>> +		goto found;
> 
> This is dangerous because it mixes actual DMAable alias PFNs with the
> device private fake PFNs.  Maybe your hardware / driver can handle
> it, but just leaking this out is not a good idea.
>

In the current implementation, regular pci_p2p pages are returned as-is 
from hmm_range_fault() - for virtual address backed by pci_p2p page, it 
will return the corresponding PFN.
That said, we can mark these via the hmm_pfn output flags so the caller 
can handle them appropriately.

>> +		    hmm_handle_device_private(range, pfn_req_flags, entry, hmm_pfn))
> 
> Overly long line here.
> 

will be fixed (v3)