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Date:   Thu, 31 Jan 2019 09:02:03 +0100
From:   Christoph Hellwig <hch@lst.de>
To:     Jerome Glisse <jglisse@redhat.com>
Cc:     Logan Gunthorpe <logang@deltatee.com>,
        Jason Gunthorpe <jgg@mellanox.com>,
        Christoph Hellwig <hch@lst.de>,
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Subject: Re: [RFC PATCH 3/5] mm/vma: add support for peer to peer to device
 vma
Message-ID: <20190131080203.GA26495@lst.de>
References: <20190129174728.6430-4-jglisse@redhat.com> <ae928aa5-a659-74d5-9734-15dfefafd3ea@deltatee.com> <20190129191120.GE3176@redhat.com> <20190129193250.GK10108@mellanox.com> <99c228c6-ef96-7594-cb43-78931966c75d@deltatee.com> <20190129205827.GM10108@mellanox.com> <20190130080208.GC29665@lst.de> <20190130174424.GA17080@mellanox.com> <bcbdfae6-cfc6-c34f-4ff2-7bb9a08f38af@deltatee.com> <20190130185027.GC5061@redhat.com>
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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..