linux-kernel - Re: Enabling peer to peer device transactions for PCIe devices

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Date:   Mon, 28 Nov 2016 14:35:08 -0500
From:   Serguei Sagalovitch <serguei.sagalovitch@....com>
To:     Logan Gunthorpe <logang@...tatee.com>,
        Jason Gunthorpe <jgunthorpe@...idianresearch.com>,
        Haggai Eran <haggaie@...lanox.com>
CC:     Christian König <christian.koenig@....com>,
        Dan Williams <dan.j.williams@...el.com>,
        "Deucher, Alexander" <Alexander.Deucher@....com>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...1.01.org>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "Kuehling, Felix" <Felix.Kuehling@....com>,
        "Bridgman, John" <John.Bridgman@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "Sander, Ben" <ben.sander@....com>,
        "Suthikulpanit, Suravee" <Suravee.Suthikulpanit@....com>,
        "Blinzer, Paul" <Paul.Blinzer@....com>,
        "Linux-media@...r.kernel.org" <Linux-media@...r.kernel.org>,
        Max Gurtovoy <maxg@...lanox.com>
Subject: Re: Enabling peer to peer device transactions for PCIe devices

On 2016-11-28 01:20 PM, Logan Gunthorpe wrote:
>
> On 28/11/16 09:57 AM, Jason Gunthorpe wrote:
>>> On PeerDirect, we have some kind of a middle-ground solution for pinning
>>> GPU memory. We create a non-ODP MR pointing to VRAM but rely on
>>> user-space and the GPU not to migrate it. If they do, the MR gets
>>> destroyed immediately.
>> That sounds horrible. How can that possibly work? What if the MR is
>> being used when the GPU decides to migrate? I would not support that
>> upstream without a lot more explanation..
> Yup, this was our experience when playing around with PeerDirect. There
> was nothing we could do if the GPU decided to invalidate the P2P
> mapping.
As soon as PeerDirect mapping is called then GPU must not "move" the
such memory.  It is by PeerDirect design. It is similar how it is works
with system memory and RDMA MR: when "get_user_pages" is called then the
memory is pinned.