linux-kernel - Re: [RFC RESEND 00/16] Split IOMMU DMA mapping operation to two steps

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Date: Mon, 25 Mar 2024 00:16:11 +0100
From: Christoph Hellwig <hch@....de>
To: Leon Romanovsky <leon@...nel.org>
Cc: Christoph Hellwig <hch@....de>, Jason Gunthorpe <jgg@...pe.ca>,
	Robin Murphy <robin.murphy@....com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
	Chaitanya Kulkarni <chaitanyak@...dia.com>,
	Jonathan Corbet <corbet@....net>, Jens Axboe <axboe@...nel.dk>,
	Keith Busch <kbusch@...nel.org>, Sagi Grimberg <sagi@...mberg.me>,
	Yishai Hadas <yishaih@...dia.com>,
	Shameer Kolothum <shameerali.kolothum.thodi@...wei.com>,
	Kevin Tian <kevin.tian@...el.com>,
	Alex Williamson <alex.williamson@...hat.com>,
	Jérôme Glisse <jglisse@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-block@...r.kernel.org, linux-rdma@...r.kernel.org,
	iommu@...ts.linux.dev, linux-nvme@...ts.infradead.org,
	kvm@...r.kernel.org, linux-mm@...ck.org,
	Bart Van Assche <bvanassche@....org>,
	Damien Le Moal <damien.lemoal@...nsource.wdc.com>,
	Amir Goldstein <amir73il@...il.com>,
	"josef@...icpanda.com" <josef@...icpanda.com>,
	"Martin K. Petersen" <martin.petersen@...cle.com>,
	"daniel@...earbox.net" <daniel@...earbox.net>,
	Dan Williams <dan.j.williams@...el.com>,
	"jack@...e.com" <jack@...e.com>, Zhu Yanjun <zyjzyj2000@...il.com>
Subject: Re: [RFC RESEND 00/16] Split IOMMU DMA mapping operation to two
 steps

On Fri, Mar 22, 2024 at 07:46:17PM +0200, Leon Romanovsky wrote:
> > As far as I can tell it totally misses the point.  Which is not to never
> > return non-P2P if the flag is set, but to return either all P2P or non-P2
> > P and not create a boundary in the single call.
> 
> You are treating FOLL_PCI_P2PDMA as a hint, but in iov_iter_extract_user_pages()
> you set it only for p2p queues. I was under impression that you want only p2p pages
> in these queues.

FOLL_PCI_P2PDMA is an indicator that the caller can cope with P2P
pages.  Most callers simply can't.