linux-kernel - Re: [PATCH 02/10] PCI/P2PDMA: Introduce p2pdma_provider structure for cleaner abstraction

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Message-ID: <20250729085336.GG402218@unreal>
Date: Tue, 29 Jul 2025 11:53:36 +0300
From: Leon Romanovsky <leon@...nel.org>
To: Christoph Hellwig <hch@....de>
Cc: Jason Gunthorpe <jgg@...dia.com>,
	Alex Williamson <alex.williamson@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Christian König <christian.koenig@....com>,
	dri-devel@...ts.freedesktop.org, iommu@...ts.linux.dev,
	Jens Axboe <axboe@...nel.dk>,
	Jérôme Glisse <jglisse@...hat.com>,
	Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
	linaro-mm-sig@...ts.linaro.org, linux-block@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-media@...r.kernel.org,
	linux-mm@...ck.org, linux-pci@...r.kernel.org,
	Logan Gunthorpe <logang@...tatee.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Robin Murphy <robin.murphy@....com>,
	Sumit Semwal <sumit.semwal@...aro.org>,
	Vivek Kasireddy <vivek.kasireddy@...el.com>,
	Will Deacon <will@...nel.org>
Subject: Re: [PATCH 02/10] PCI/P2PDMA: Introduce p2pdma_provider structure
 for cleaner abstraction

On Tue, Jul 29, 2025 at 09:52:09AM +0200, Christoph Hellwig wrote:
> On Sun, Jul 27, 2025 at 03:51:58PM -0300, Jason Gunthorpe wrote:
> > On Thu, Jul 24, 2025 at 09:59:22AM +0200, Christoph Hellwig wrote:
> > > On Thu, Jul 24, 2025 at 10:55:33AM +0300, Leon Romanovsky wrote:
> > > > Please, see last patch in the series https://lore.kernel.org/all/aea452cc27ca9e5169f7279d7b524190c39e7260.1753274085.git.leonro@nvidia.com
> > > > It gives me a way to call p2p code with stable pointer for whole BAR.
> > > > 
> > > 
> > > That simply can't work.
> > 
> > Why not?
> > 
> > That's the whole point of this, to remove struct page and use
> > something else as a handle for the p2p when doing the DMA API stuff.
> 
> Because the struct page is the only thing that:
> 
>  a) dma-mapping works on
>  b) is the only place we can discover the routing information, but also
>     more importantly ensure that the underlying page is still present
>     and the device is not hot unplugged, or in a very theoretical worst
>     case replaced by something else.

It is correct in general case, but here we are talking about MMIO
memory, which is "connected" to device X and routing information is
stable.

Thanks

> 
>