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

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Message-ID: <20241016154428.GD4020792@ziepe.ca>
Date: Wed, 16 Oct 2024 12:44:28 -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 Tue, Oct 15, 2024 at 09:49:30PM -0700, Christoph Hellwig wrote:

> > +	/*
> > +	 * Used for private (un-addressable) device memory only. Return a
> > +	 * corresponding struct page, that can be mapped to device
> > +	 * (e.g using dma_map_page)
> > +	 */
> > +	struct page *(*get_dma_page_for_device)(struct page *private_page);
> 
> We are talking about P2P memory here.  How do you manage to get a page
> that dma_map_page can be used on?  All P2P memory needs to use the P2P
> aware dma_map_sg as the pages for P2P memory are just fake zone device
> pages.

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.. I think this series
is at RFC quality, but shows more of the next steps.

I'm guessing they got their testing done so far on a system without an
iommu translation?

> which also makes it clear that returning a page from the method is
> not that great, a PFN might work a lot better, e.g.
> 
> 	unsigned long (*device_private_dma_pfn)(struct page *page);

Ideally I think we should not have the struct page * at all through
these APIs if we can avoid it..

Jason