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

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Date:   Wed, 19 Apr 2017 11:21:50 +1000
From:   Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:     Jason Gunthorpe <jgunthorpe@...idianresearch.com>,
        Dan Williams <dan.j.williams@...el.com>
Cc:     Logan Gunthorpe <logang@...tatee.com>,
        Bjorn Helgaas <helgaas@...nel.org>,
        Christoph Hellwig <hch@....de>,
        Sagi Grimberg <sagi@...mberg.me>,
        "James E.J. Bottomley" <jejb@...ux.vnet.ibm.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        Jens Axboe <axboe@...nel.dk>,
        Steve Wise <swise@...ngridcomputing.com>,
        Stephen Bates <sbates@...thlin.com>,
        Max Gurtovoy <maxg@...lanox.com>,
        Keith Busch <keith.busch@...el.com>, linux-pci@...r.kernel.org,
        linux-scsi <linux-scsi@...r.kernel.org>,
        linux-nvme@...ts.infradead.org, linux-rdma@...r.kernel.org,
        linux-nvdimm <linux-nvdimm@...1.01.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Jerome Glisse <jglisse@...hat.com>
Subject: Re: [RFC 0/8] Copy Offload with Peer-to-Peer PCI Memory

On Tue, 2017-04-18 at 15:22 -0600, Jason Gunthorpe wrote:
> On Tue, Apr 18, 2017 at 02:11:33PM -0700, Dan Williams wrote:
> > > I think this opens an even bigger can of worms..
> > 
> > No, I don't think it does. You'd only shim when the target page is
> > backed by a device, not host memory, and you can figure this out by
> > a
> > is_zone_device_page()-style lookup.
> 
> The bigger can of worms is how do you meaningfully stack dma_ops.
> 
> What does the p2p provider do when it detects a p2p page?

Yeah I think we don't really want to stack dma_ops... thinking more
about it.

As I just wrote, it looks like we might need a more specialised hook
in the devmap to be used by the main dma_op, on a per-page basis.

Ben.