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    SubjectRe: [PATCH v2 00/10] Copy Offload in NVMe Fabrics with P2P PCI Memory
    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 = page->pgmap->data;
    ppfn = 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,
    Jérôme

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