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

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Message-ID: <aPiDa-QruoHC3alk@infradead.org>
Date: Wed, 22 Oct 2025 00:10:35 -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 Mon, Oct 20, 2025 at 09:58:54AM -0300, Jason Gunthorpe wrote:
> I explained it in detail in the message you are repling to. If
> something is not clear can you please be more specific??
> 
> Is it the mmap in VFIO perhaps that is causing these questions?
> 
> VFIO uses a PFNMAP VMA, so you can't pin_user_page() it. It uses
> unmap_mapping_range() during its remove() path to get rid of the VMA
> PTEs.

This all needs to g• into the explanation.

> Instead the DMABUF FD is used to pass the MMIO pages between VFIO and
> another driver. DMABUF has a built in invalidation mechanism that VFIO
> triggers before remove(). The invalidation removes access from the
> other driver.
> 
> This is different than NVMe which has no invalidation. NVMe does
> unmap_mapping_range() on the VMA and waits for all the short lived
> pgmap references to clear. We don't need anything like that because
> DMABUF invalidation is synchronous.

Please add documentation for this model to the source tree.