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

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Message-ID: <20250727185158.GE7551@nvidia.com>
Date: Sun, 27 Jul 2025 15:51:58 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Christoph Hellwig <hch@....de>
Cc: Leon Romanovsky <leon@...nel.org>,
	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 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.

The caller must make sure the lifetimes all work out. The handle must
live longer than any active DMAs, etc, etc. DMABUF with invalidation
lets vfio do that.

This is why the DMA api code was taught to use phys_addr_t and not
touch the struct page so it could work with struct-pageless memory.

The idea was to end up with two layers in the P2P code where the lower
layer only works on the handle, and then there is an optional struct
page/genalloc/etc layer for places that want struct page and mmap.

Jason