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

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Message-ID: <aH3lPnIUGn29HJFo@infradead.org>
Date: Sun, 20 Jul 2025 23:59:10 -0700
From: Christoph Hellwig <hch@...radead.org>
To: Yonatan Maman <ymaman@...dia.com>
Cc: Jérôme Glisse <jglisse@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jason Gunthorpe <jgg@...pe.ca>, Leon Romanovsky <leon@...nel.org>,
	Lyude Paul <lyude@...hat.com>, Danilo Krummrich <dakr@...nel.org>,
	David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
	Alistair Popple <apopple@...dia.com>,
	Ben Skeggs <bskeggs@...dia.com>,
	Michael Guralnik <michaelgur@...dia.com>,
	Or Har-Toov <ohartoov@...dia.com>,
	Daisuke Matsuda <dskmtsd@...il.com>, Shay Drory <shayd@...dia.com>,
	linux-mm@...ck.org, linux-rdma@...r.kernel.org,
	dri-devel@...ts.freedesktop.org, nouveau@...ts.freedesktop.org,
	linux-kernel@...r.kernel.org, Gal Shalom <GalShalom@...dia.com>
Subject: Re: [PATCH v2 1/5] mm/hmm: HMM API to enable P2P DMA for device
 private pages

On Fri, Jul 18, 2025 at 02:51:08PM +0300, Yonatan Maman wrote:
> From: Yonatan Maman <Ymaman@...dia.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.