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 <yma...@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)
