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

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Date:   Wed, 19 Apr 2017 10:41:42 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Jerome Glisse <jglisse@...hat.com>
Cc:     Logan Gunthorpe <logang@...tatee.com>,
        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>
Subject: Re: [RFC 0/8] Copy Offload with Peer-to-Peer PCI Memory

On Wed, Apr 19, 2017 at 10:32 AM, Jerome Glisse <jglisse@...hat.com> wrote:
> On Wed, Apr 19, 2017 at 10:01:23AM -0700, Dan Williams wrote:
>> 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.
>
> Dan you asked me to not use devm_memremap_pages() because you didn't
> want to have HMM memory in the pgmap_radix, did you change opinion
> on that ? :)

No, not quite ;-). I still don't think we should require the non-HMM
to pass NULL for all the HMM arguments. What I like about Logan's
proposal is to have a separate create and register steps dev_pagemap.
That way call paths that don't care about HMM specifics can just turn
around and register the vanilla dev_pagemap.

> Note i won't make any change now on that front but if it make sense
> i am happy to do it as separate patchset on top of HMM.
>
> Also i don't want p2pmem to be an exclusive or with HMM, we will want
> GPU to do peer to peer DMA and thus HMM ZONE_DEVICE pages to support
> this too.

Yes, this makes sense I think we really just want to distinguish host
memory or not in terms of the dev_pagemap type.