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

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Date:   Wed, 19 Apr 2017 10:01:23 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Logan Gunthorpe <logang@...tatee.com>
Cc:     Jason Gunthorpe <jgunthorpe@...idianresearch.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        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 Wed, Apr 19, 2017 at 9:48 AM, Logan Gunthorpe <logang@...tatee.com> wrote:
>
>
> On 19/04/17 09:55 AM, Jason Gunthorpe wrote:
>> I was thinking only this one would be supported with a core code
>> helper..
>
> Pivoting slightly: I was looking at how HMM uses ZONE_DEVICE. They add a
> type flag to the dev_pagemap structure which would be very useful to us.
> We could add another MEMORY_DEVICE_P2P type to distinguish p2p pages.
> Then, potentially, we could add a dma_map callback to the structure
> (possibly unioned with an hmm field). The dev_ops providers would then
> just need to do something like this (enclosed in a helper):
>
> if (is_zone_device_page(page)) {
>         pgmap = get_dev_pagemap(page_to_pfn(page));
>         if (!pgmap || pgmap->type !=  MEMORY_DEVICE_P2P ||
>             !pgmap->dma_map)
>                 return 0;
>
>         dma_addr = pgmap->dma_map(dev, pgmap->dev, page);
>         put_dev_pagemap(pgmap);
>         if (!dma_addr)
>                 return 0;
>         ...
> }
>
> The pci_enable_p2p_bar function would then just need to call
> devm_memremap_pages with the dma_map callback set to a function that
> does the segment check and the offset calculation.
>
> Thoughts?
>
> @Jerome: my feedback to you would be that your patch assumes all users
> of devm_memremap_pages are MEMORY_DEVICE_PERSISTENT. It would be more
> useful if it was generic. My suggestion would be to have the caller
> allocate the dev_pagemap structure, populate it and pass it into
> devm_memremap_pages. Given that pretty much everything in that structure
> are already arguments to that function, I feel like this makes sense.
> This should also help to unify hmm_devmem_pages_create and
> devm_memremap_pages which look very similar to each other.

I like that change. Also the types should describe the memory relative
to its relationship to struct page, not whether it is persistent or
not. I would consider volatile and persistent memory that is attached
to the cpu memory controller and i/o coherent as the same type of
memory. DMA incoherent ranges like P2P and HMM should get their own
types.