Re: [PATCH v2 00/10] Copy Offload in NVMe Fabrics with P2P PCI Memory

[Jerome Glisse]

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