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

[Christian König]

Am 23.11.22 um 13:46 schrieb 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..

Well not only out of tree drivers. The intree KVM got that horrible 
wrong as well, those where the latest guys complaining about it.

>
> 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..

That's not sufficient. The pages are released much later than things 
actually go wrong. In most cases this WARN_ON here won't hit.

Christian.

>
> Jason