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Date: Sat, 13 Jul 2024 07:24:08 +0200
From: Christoph Hellwig <hch@lst.de>
To: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Christoph Hellwig <hch@lst.de>, Leon Romanovsky <leon@kernel.org>,
	Jens Axboe <axboe@kernel.dk>, Robin Murphy <robin.murphy@arm.com>,
	Joerg Roedel <joro@8bytes.org>, Will Deacon <will@kernel.org>,
	Keith Busch <kbusch@kernel.org>, "Zeng, Oak" <oak.zeng@intel.com>,
	Chaitanya Kulkarni <kch@nvidia.com>,
	Sagi Grimberg <sagi@grimberg.me>,
	Bjorn Helgaas <bhelgaas@google.com>,
	Logan Gunthorpe <logang@deltatee.com>,
	Yishai Hadas <yishaih@nvidia.com>,
	Shameer Kolothum <shameerali.kolothum.thodi@huawei.com>,
	Kevin Tian <kevin.tian@intel.com>,
	Alex Williamson <alex.williamson@redhat.com>,
	Marek Szyprowski <m.szyprowski@samsung.com>,
	=?iso-8859-1?B?Suly9G1l?= Glisse <jglisse@redhat.com>,
	Andrew Morton <akpm@linux-foundation.org>,
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	linux-nvme@lists.infradead.org, linux-pci@vger.kernel.org,
	kvm@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [RFC PATCH v1 00/18] Provide a new two step DMA API mapping API
Message-ID: <20240713052408.GA25890@lst.de>
References: <cover.1719909395.git.leon@kernel.org> <20240705063910.GA12337@lst.de> <20240708235721.GF14050@ziepe.ca> <20240709062015.GB16180@lst.de> <20240709190320.GN14050@ziepe.ca> <20240710062212.GA25895@lst.de> <20240711232917.GR14050@ziepe.ca> <20240712045422.GA4774@lst.de> <20240712124237.GX14050@ziepe.ca>
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User-Agent: Mutt/1.5.17 (2007-11-01)

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.