From mboxrd@z Thu Jan  1 00:00:00 1970
From: Jerome Glisse <jglisse-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
Subject: Re: [PATCH v2 00/10] Copy Offload in NVMe Fabrics with P2P PCI Memory
Date: Thu, 1 Mar 2018 16:18:18 -0500
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To: Logan Gunthorpe <logang-OTvnGxWRz7hWk0Htik3J/w@public.gmane.org>
Cc: Jens Axboe <axboe-tSWWG44O7X1aa/9Udqfwiw@public.gmane.org>, Keith Busch <keith.busch-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>, Oliver OHalloran <oliveroh-8fk3Idey6ehBDgjK7y7TUQ@public.gmane.org>, benh-8fk3Idey6ehBDgjK7y7TUQ@public.gmane.org, linux-nvdimm <linux-nvdimm-hn68Rpc1hR1g9hUCZPvPmw@public.gmane.org>, linux-rdma <linux-rdma-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>, linux-pci-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Linux Kernel Mailing List <linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>, linux-nvme-IAPFreCvJWM7uuMidbF8XUB+6BGkLq7r@public.gmane.org, linux-block-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Alex Williamson <alex.williamson-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>, Jason Gunthorpe <jgg-VPRAkNaXOzVWk0Htik3J/w@public.gmane.org>, Bjorn Helgaas <bhelgaas-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>, Max Gurtovoy <maxg-VPRAkNaXOzVWk0Htik3J/w@public.gmane.org>, Christoph Hellwig <hch-jcswGhMUV9g@public.gmane.org>
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On Thu, Mar 01, 2018 at 02:11:34PM -0700, Logan Gunthorpe wrote:
> =

> =

> On 01/03/18 02:03 PM, Benjamin Herrenschmidt wrote:
> > However, what happens if anything calls page_address() on them ? Some
> > DMA ops do that for example, or some devices might ...
> =

> Although we could probably work around it with some pain, we rely on
> page_address() and virt_to_phys(), etc to work on these pages. So on x86,
> yes, it makes it into the linear mapping.

This is pretty easy to do with HMM:

unsigned long hmm_page_to_phys_pfn(struct page *page)
{
    struct hmm_devmem *devmem;
    unsigned long ppfn;

    /* Sanity test maybe BUG_ON() */
    if (!is_device_private_page(page))
        return -1UL;

    devmem =3D page->pgmap->data;
    ppfn =3D page_to_page(page) - devmem->pfn_first;
    return ppfn + devmem->device_phys_base_pfn;
}

Note that last field does not exist in today HMM because i did not need
such helper so far but this can be added.

Cheers,
J=E9r=F4me