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

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Date:   Wed, 23 Nov 2016 14:06:34 -0500
From:   Serguei Sagalovitch <serguei.sagalovitch@....com>
To:     Bart Van Assche <bart.vanassche@...disk.com>,
        Logan Gunthorpe <logang@...tatee.com>,
        Dan Williams <dan.j.williams@...el.com>,
        "Deucher, Alexander" <Alexander.Deucher@....com>
CC:     "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.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>,
        "Koenig, Christian" <Christian.Koenig@....com>,
        "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>,
        <haggaie@...lanox.com>
Subject: Re: Enabling peer to peer device transactions for PCIe devices

On 2016-11-23 12:27 PM, Bart Van Assche wrote:
> On 11/23/2016 09:13 AM, Logan Gunthorpe wrote:
>> IMO any memory that has been registered for a P2P transaction should be
>> locked from being evicted. So if there's a get_user_pages call it needs
>> to be pinned until the put_page. The main issue being with the RDMA
>> case: handling an eviction when a chunk of memory has been registered as
>> an MR would be very tricky. The MR may be relied upon by another host
>> and the kernel would have to inform user-space the MR was invalid then
>> user-space would have to tell the remote application.
>
> Hello Logan,
>
> Are you aware that the Linux kernel already supports ODP (On Demand 
> Paging)? See also the output of git grep -nHi on.demand.paging. See 
> also 
> https://www.openfabrics.org/images/eventpresos/workshops2014/DevWorkshop/presos/Tuesday/pdf/04_ODP_update.pdf.
>
> Bart.
My understanding is that  the main problems are (a) h/w support (b) 
compatibility with IB Verbs semantic.