[PATCH] RFC: dma-buf: Require VM_SPECIAL vma for mmap

[Jason Gunthorpe]

On Thu, Feb 04, 2021 at 09:19:57PM +0100, Daniel Vetter wrote:
> On Thu, Feb 4, 2021 at 9:09 PM Jason Gunthorpe <jgg at ziepe.ca> wrote:
> >
> > On Thu, Feb 04, 2021 at 08:59:59PM +0100, Daniel Vetter wrote:
> >
> > > So I think just checking for VM_PFNMAP after the vma is set up should
> > > be enough to guarantee we'll only have pte_special ptes in there,
> > > ever. But I'm not sure, this stuff all isn't really documented much
> > > and the code is sometimes a maze (to me at least).
> >
> > Yes, that makes sense. VM_PFNMAP and !VM_MIXEDMAP seems like the right
> > check after the VMA is populated
> >
> > But how do you stuff special pfns into a VMA outside the fault
> > handler?
> 
> Many drivers we have don't have dynamic buffer management (kinda
> overkill for a few framebuffers on a display-only IP block), so the
> just remap_pfn_range on ->mmap, and don't have a fault handler at all.

remap_pfn_range() makes sense, do you expect drivers using struct page
backed memory to call that as well?

Jason