linux-kernel - Re: [PATCH v2 4/5] RDMA/mlx5: Enable P2P DMA with fallback mechanism

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Message-ID: <aIHhGi3adOiLykJn@infradead.org>
Date: Thu, 24 Jul 2025 00:30:34 -0700
From: Christoph Hellwig <hch@...radead.org>
To: Jason Gunthorpe <jgg@...pe.ca>
Cc: Christoph Hellwig <hch@...radead.org>,
	Yonatan Maman <ymaman@...dia.com>,
	Jérôme Glisse <jglisse@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	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 4/5] RDMA/mlx5: Enable P2P DMA with fallback mechanism

On Wed, Jul 23, 2025 at 12:55:22AM -0300, Jason Gunthorpe wrote:
> On Mon, Jul 21, 2025 at 12:03:41AM -0700, Christoph Hellwig wrote:
> > On Fri, Jul 18, 2025 at 02:51:11PM +0300, Yonatan Maman wrote:
> > > From: Yonatan Maman <Ymaman@...dia.com>
> > > 
> > > Add support for P2P for MLX5 NIC devices with automatic fallback to
> > > standard DMA when P2P mapping fails.
> > 
> > That's now how the P2P API works.  You need to check the P2P availability
> > higher up.
> 
> How do you mean?
> 
> This looks OKish to me, for ODP and HMM it has to check the P2P
> availability on a page by page basis because every single page can be
> a different origin device.
> 
> There isn't really a higher up here...

The DMA API expects the caller to already check for connectability,
why can't HMM do that like everyone else?