linux-kernel - Re: [PATCH v1 1/4] mm/hmm: HMM API for P2P DMA to device zone pages

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Message-ID: <20241017130539.GA897978@ziepe.ca>
Date: Thu, 17 Oct 2024 10:05:39 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: Christoph Hellwig <hch@...radead.org>
Cc: Yonatan Maman <ymaman@...dia.com>, nouveau@...ts.freedesktop.org,
	linux-kernel@...r.kernel.org, linux-rdma@...r.kernel.org,
	linux-mm@...ck.org, herbst@...hat.com, lyude@...hat.com,
	dakr@...hat.com, airlied@...il.com, simona@...ll.ch,
	leon@...nel.org, jglisse@...hat.com, akpm@...ux-foundation.org,
	dri-devel@...ts.freedesktop.org, apopple@...dia.com,
	bskeggs@...dia.com, Gal Shalom <GalShalom@...dia.com>
Subject: Re: [PATCH v1 1/4] mm/hmm: HMM API for P2P DMA to device zone pages

On Thu, Oct 17, 2024 at 04:58:12AM -0700, Christoph Hellwig wrote:
> On Wed, Oct 16, 2024 at 02:44:45PM -0300, Jason Gunthorpe wrote:
> > > > FWIW, I've been expecting this series to be rebased on top of Leon's
> > > > new DMA API series so it doesn't have this issue..
> > > 
> > > That's not going to make a difference at this level.
> > 
> > I'm not sure what you are asking then.
> > 
> > Patch 2 does pci_p2pdma_add_resource() and so a valid struct page with
> > a P2P ZONE_DEVICE type exists, and that gets returned back to the
> > hmm/odp code.
> > 
> > Today odp calls dma_map_page() which only works by chance in limited
> > cases. With Leon's revision it will call hmm_dma_map_pfn() ->
> > dma_iova_link() which does call pci_p2pdma_map_type() and should do
> > the right thing.
> 
> Again none of this affects the code posted here.  It reshuffles the
> callers but has no direct affect on the patches posted here.

I didn't realize till last night that Leon's series did not have P2P
support.

What I'm trying to say is that this is a multi-series project.

A followup based on Leon's initial work will get the ODP DMA mapping
path able to support ZONE_DEVICE P2P pages.

Once that is done, this series sits on top. This series is only about
hmm and effectively allows hmm_range_fault() to return a ZONE_DEVICE
P2P page.

Yonatan should explain this better in the cover letter and mark it as
a RFC series.

So, I know we are still figuring out the P2P support on the DMA API
side, but my expectation for hmm is that hmm_range_fault() returing a
ZONE_DEVICE P2P page is going to be what we want.

> (and the current DMA series lacks P2P support, I'm trying to figure
> out how to properly handle it at the moment).

Yes, I see, I looked through those patches last night and there is a
gap there.

Broadly I think whatever flow NVMe uses for P2P will apply to ODP as
well.

Thanks,
Jason