On Fri, Jul 18, 2025 at 11:44:42AM -0300, Jason Gunthorpe wrote:
> On Fri, Jul 18, 2025 at 03:17:00PM +0100, Matthew Wilcox wrote:
> > On Fri, Jul 18, 2025 at 02:51:08PM +0300, Yonatan Maman wrote:
> > > +++ b/include/linux/memremap.h
> > > @@ -89,6 +89,14 @@ struct dev_pagemap_ops {
> > > */
> > > vm_fault_t (*migrate_to_ram)(struct vm_fault *vmf);
> > >
> > > + /*
> > > + * Used for private (un-addressable) device memory only. Return a
> > > + * corresponding PFN for a page that can be mapped to device
> > > + * (e.g using dma_map_page)
> > > + */
> > > + int (*get_dma_pfn_for_device)(struct page *private_page,
> > > + unsigned long *dma_pfn);
> >
> > This makes no sense. If a page is addressable then it has a PFN.
> > If a page is not addressable then it doesn't have a PFN.
>
> The DEVICE_PRIVATE pages have a PFN, but it is not usable for
> anything.
OK, then I don't understand what DEVICE PRIVATE means.
I thought it was for memory on a PCIe device that isn't even visible
through a BAR and so the CPU has no way of addressing it directly.
But now you say that it has a PFN, which means it has a physical
address, which means it's accessible to the CPU.
So what is it?
> This is effectively converting from a DEVICE_PRIVATE page to an actual
> DMA'able address of some kind. The DEVICE_PRIVATE is just a non-usable
> proxy, like a swap entry, for where the real data is sitting.
>
> Jason
>
