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

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Date:   Mon, 28 Nov 2016 09:57:51 -0700
From:   Jason Gunthorpe <jgunthorpe@...idianresearch.com>
To:     Haggai Eran <haggaie@...lanox.com>
Cc:     Christian König <christian.koenig@....com>,
        Logan Gunthorpe <logang@...tatee.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 Sun, Nov 27, 2016 at 04:02:16PM +0200, Haggai Eran wrote:

> > Like in ODP, MMU notifiers/HMM are used to monitor for translation
> > changes. If a change comes in the GPU driver checks if an executing
> > command is touching those pages and blocks the MMU notifier until the
> > command flushes, then unfaults the page (blocking future commands) and
> > unblocks the mmu notifier.

> I think blocking mmu notifiers against something that is basically
> controlled by user-space can be problematic. This can block things like
> memory reclaim. If you have user-space access to the device's queues,
> user-space can block the mmu notifier forever.

Right, I mentioned that..

> 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..

I know people don't like requiring new hardware, but in this case we
really do need ODP hardware to get all the semantics people want..

> Another thing I think is that while HMM is good for user-space
> applications, for kernel p2p use there is no need for that. Using

>From what I understand we are not really talking about kernel p2p,
everything proposed so far is being mediated by a userspace VMA, so
I'd focus on making that work.

Jason