linux-kernel - Re: [RFC PATCH 3/5] mm/vma: add support for peer to peer to device vma

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Message-ID: <20190131080203.GA26495@lst.de>
Date:   Thu, 31 Jan 2019 09:02:03 +0100
From:   Christoph Hellwig <hch@....de>
To:     Jerome Glisse <jglisse@...hat.com>
Cc:     Logan Gunthorpe <logang@...tatee.com>,
        Jason Gunthorpe <jgg@...lanox.com>,
        Christoph Hellwig <hch@....de>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Christian Koenig <christian.koenig@....com>,
        Felix Kuehling <Felix.Kuehling@....com>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>,
        Joerg Roedel <jroedel@...e.de>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>
Subject: Re: [RFC PATCH 3/5] mm/vma: add support for peer to peer to device
 vma

On Wed, Jan 30, 2019 at 01:50:27PM -0500, Jerome Glisse wrote:
> I do not see how VMA changes are any different than using struct page
> in respect to userspace exposure. Those vma callback do not need to be
> set by everyone, in fact expectation is that only handful of driver
> will set those.
> 
> How can we do p2p between RDMA and GPU for instance, without exposure
> to userspace ? At some point you need to tell userspace hey this kernel
> does allow you to do that :)

To do RDMA on a memory region you need struct page backіng to start
with..