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

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Date:   Mon, 28 Nov 2016 11:20:27 -0700
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Jason Gunthorpe <jgunthorpe@...idianresearch.com>,
        Haggai Eran <haggaie@...lanox.com>
Cc:     Christian König <christian.koenig@....com>,
        Serguei Sagalovitch <serguei.sagalovitch@....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 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. It just meant the application would fail or need complicated
logic to detect this and redo just about everything. And given that it
was a reasonably rare occurrence during development it probably means
not a lot of applications will be developed to handle it and most would
end up being randomly broken in environments with memory pressure.

Logan