[Linaro-mm-sig] Re: [PATCH] dma-buf: Require VM_PFNMAP vma for mmap

[Jason Gunthorpe]

On Wed, Nov 23, 2022 at 11:06:55AM +0100, Daniel Vetter wrote:

> > Maybe a GFP flag to set the page reference count to zero or something
> > like this?
> 
> Hm yeah that might work. I'm not sure what it will all break though?
> And we'd need to make sure that underflowing the page refcount dies in
> a backtrace.

Mucking with the refcount like this to protect against crazy out of
tree drives seems horrible..

The WARN_ON(pag_count(p) != 1) seems like a reasonable thing to do
though, though you must combine this with the special PTE flag..

Jason