linux-kernel - Re: [RFC 0/8] Copy Offload with Peer-to-Peer PCI Memory

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Date:   Wed, 19 Apr 2017 17:07:58 -0700
From:   Dan Williams <dan.j.williams@...il.com>
To:     Logan Gunthorpe <logang@...tatee.com>
Cc:     Jason Gunthorpe <jgunthorpe@...idianresearch.com>,
        Jens Axboe <axboe@...nel.dk>,
        "James E.J. Bottomley" <jejb@...ux.vnet.ibm.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        linux-rdma@...r.kernel.org,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Steve Wise <swise@...ngridcomputing.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-nvme@...ts.infradead.org,
        Keith Busch <keith.busch@...el.com>,
        Jerome Glisse <jglisse@...hat.com>,
        Bjorn Helgaas <helgaas@...nel.org>, linux-pci@...r.kernel.org,
        linux-nvdimm <linux-nvdimm@...1.01.org>,
        Max Gurtovoy <maxg@...lanox.com>,
        linux-scsi <linux-scsi@...r.kernel.org>,
        Christoph Hellwig <hch@....de>
Subject: Re: [RFC 0/8] Copy Offload with Peer-to-Peer PCI Memory

On Wed, Apr 19, 2017 at 3:55 PM, Logan Gunthorpe <logang@...tatee.com> wrote:
>
>
> On 19/04/17 02:48 PM, Jason Gunthorpe wrote:
>> On Wed, Apr 19, 2017 at 01:41:49PM -0600, Logan Gunthorpe wrote:
>>
>>>> But.. it could point to a GPU and the GPU struct device could have a
>>>> proxy dma_ops like Dan pointed out.
>>>
>>> Seems a bit awkward to me that in order for the intended use case, you
>>> have to proxy the dma_ops. I'd probably still suggest throwing a couple
>>> ops for things like this in the dev_pagemap.
>>
>> Another option is adding a new 'struct completer_dma_ops *' to struct
>> device for this use case.
>>
>> Seems like a waste to expand dev_pagemap when we only need a unique
>> value per struct device?
>
> I feel like expanding dev_pagemap has a much lower impact than expanding
> struct device... dev_pagemap is only one instance per zone device region
> so expanding it shouldn't be a huge issue. Expanding struct device means
> every device struct in the system gets bigger.

Especially since we expect a very small subset of devices will ever support p2p.