[PATCH v4 21/23] mm: use custom page_free for P2PDMA pages

[Christoph Hellwig]

On Wed, Nov 17, 2021 at 02:54:08PM -0700, Logan Gunthorpe wrote:
> When P2PDMA pages are passed to userspace, they will need to be
> reference counted properly and returned to their genalloc after their
> reference count returns to 1. This is accomplished with the existing
> DEV_PAGEMAP_OPS and the .page_free() operation.
> 
> Change CONFIG_P2PDMA to select CONFIG_DEV_PAGEMAP_OPS and add
> MEMORY_DEVICE_PCI_P2PDMA to page_is_devmap_managed(),
> devmap_managed_enable_[put|get]() and free_devmap_managed_page().

Uuuh.  We are trying hard to kill off this magic free at refcount 1
behavior in the amdgpu device coherent series.  We really should not
add more of this.