[PATCH v2 1/5] mm/hmm: HMM API to enable P2P DMA for device private pages
Christoph Hellwig
hch at infradead.org
Mon Jul 21 06:59:10 UTC 2025
On Fri, Jul 18, 2025 at 02:51:08PM +0300, Yonatan Maman wrote:
> From: Yonatan Maman <Ymaman at 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.
> + /*
> + * 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.
> + hmm_handle_device_private(range, pfn_req_flags, entry, hmm_pfn))
Overly long line here.
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