linux-kernel - Re: [RFC PATCH v1 00/18] Provide a new two step DMA API mapping API

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Message-ID: <20240713052408.GA25890@lst.de>
Date: Sat, 13 Jul 2024 07:24:08 +0200
From: Christoph Hellwig <hch@....de>
To: Jason Gunthorpe <jgg@...pe.ca>
Cc: Christoph Hellwig <hch@....de>, Leon Romanovsky <leon@...nel.org>,
	Jens Axboe <axboe@...nel.dk>, Robin Murphy <robin.murphy@....com>,
	Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
	Keith Busch <kbusch@...nel.org>, "Zeng, Oak" <oak.zeng@...el.com>,
	Chaitanya Kulkarni <kch@...dia.com>,
	Sagi Grimberg <sagi@...mberg.me>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Logan Gunthorpe <logang@...tatee.com>,
	Yishai Hadas <yishaih@...dia.com>,
	Shameer Kolothum <shameerali.kolothum.thodi@...wei.com>,
	Kevin Tian <kevin.tian@...el.com>,
	Alex Williamson <alex.williamson@...hat.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Jérôme Glisse <jglisse@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-rdma@...r.kernel.org, iommu@...ts.linux.dev,
	linux-nvme@...ts.infradead.org, linux-pci@...r.kernel.org,
	kvm@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RFC PATCH v1 00/18] Provide a new two step DMA API mapping API

On Fri, Jul 12, 2024 at 09:42:37AM -0300, Jason Gunthorpe wrote:
> On Fri, Jul 12, 2024 at 06:54:22AM +0200, Christoph Hellwig wrote:
> 
> > This is all purely hypothetical, and I'm happy to just check for it
> > and reject it for it now.
> 
> I do know a patch set is cooking to allow mixing ZONE_DEVICE P2P and
> anon memory in the same VMA ala HMM with transparent migration of
> ZONE_DEVICE to anon.
> 
> In this situation userspace will be generating IO with no idea about
> any P2P/!P2P boundaries.

Yes.  And as said from the beginning of these discussion I think the
right way is to change the gup code so that for a single call to
get/pin_user_pages it always returns either only P2P pages or non-P2P
pages only, with the FOLL_PCI_P2PDMA just allowing P2P pages at all.
Similarly they could easily just return one kind of P2P pages per
call only.