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

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Date:   Wed, 19 Apr 2017 10:48:51 -0600
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Jason Gunthorpe <jgunthorpe@...idianresearch.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:     Dan Williams <dan.j.williams@...el.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 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.

Logan