linux-kernel - Re: [PATCH v5 1/9] PCI/P2PDMA: Separate the mmap() support from the core logic

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Message-ID: <aPYqliGwJTcZznSX@infradead.org>
Date: Mon, 20 Oct 2025 05:27:02 -0700
From: Christoph Hellwig <hch@...radead.org>
To: Jason Gunthorpe <jgg@...dia.com>
Cc: Christoph Hellwig <hch@...radead.org>,
	Leon Romanovsky <leon@...nel.org>,
	Alex Williamson <alex.williamson@...hat.com>,
	Leon Romanovsky <leonro@...dia.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>, 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 v5 1/9] PCI/P2PDMA: Separate the mmap() support from the
 core logic

On Fri, Oct 17, 2025 at 08:53:20AM -0300, Jason Gunthorpe wrote:
> On Thu, Oct 16, 2025 at 11:30:06PM -0700, Christoph Hellwig wrote:
> > On Mon, Oct 13, 2025 at 06:26:03PM +0300, Leon Romanovsky wrote:
> > > The DMA API now has a new flow, and has gained phys_addr_t support, so
> > > it no longer needs struct pages to perform P2P mapping.
> > 
> > That's news to me.  All the pci_p2pdma_map_state machinery is still
> > based on pgmaps and thus pages.
> 
> We had this discussion already three months ago:
> 
> https://lore.kernel.org/all/20250729131502.GJ36037@nvidia.com/
> 
> These couple patches make the core pci_p2pdma_map_state machinery work
> on struct p2pdma_provider, and pgmap is just one way to get a
> p2pdma_provider *
> 
> The struct page paths through pgmap go page->pgmap->mem to get
> p2pdma_provider.
> 
> The non-struct page paths just have a p2pdma_provider * without a
> pgmap. In this series VFIO uses
> 
> +	*provider = pcim_p2pdma_provider(pdev, bar);
> 
> To get the provider for a specific BAR.

And what protects that life time?  I've not seen anyone actually
building the proper lifetime management.  And if someone did the patches
need to clearly point to that.

> I think I've answered this three times now - for DMABUF the DMABUF
> invalidation scheme is used to control the lifetime and no DMA mapping
> outlives the provider, and the provider doesn't outlive the driver.

How?

> Obviously you cannot use the new p2provider mechanism without some
> kind of protection against use after hot unplug, but it doesn't have
> to be struct page based.

And how does this interact with everyone else expecting pgmap based
lifetime management.