[PATCH] dma-buf: Require VM_PFNMAP vma for mmap

[Daniel Vetter]

On Wed, 23 Nov 2022 at 10:06, Christian König <christian.koenig at amd.com> wrote:
> Am 22.11.22 um 20:50 schrieb Daniel Vetter:
> > On Tue, 22 Nov 2022 at 20:34, Jason Gunthorpe <jgg at ziepe.ca> wrote:
> >> On Tue, Nov 22, 2022 at 08:29:05PM +0100, Daniel Vetter wrote:
> >>> You nuke all the ptes. Drivers that move have slightly more than a
> >>> bare struct file, they also have a struct address_space so that
> >>> invalidate_mapping_range() works.
> >> Okay, this is one of the ways that this can be made to work correctly,
> >> as long as you never allow GUP/GUP_fast to succeed on the PTEs. (this
> >> was the DAX mistake)
> > Hence this patch, to enforce that no dma-buf exporter gets this wrong.
> > Which some did, and then blamed bug reporters for the resulting splats
> > :-) One of the things we've reverted was the ttm huge pte support,
> > since that doesn't have the pmd_special flag (yet) and so would let
> > gup_fast through.
>
> The problem is not only gup, a lot of people seem to assume that when
> you are able to grab a reference to a page that the ptes pointing to
> that page can't change any more. And that's obviously incorrect.
>
> I witnessed tons of discussions about that already. Some customers even
> modified our code assuming that and then wondered why the heck they ran
> into data corruption.
>
> It's gotten so bad that I've even proposed intentionally mangling the
> page reference count on TTM allocated pages:
> https://patchwork.kernel.org/project/dri-devel/patch/20220927143529.135689-1-christian.koenig@amd.com/

Yeah maybe something like this could be applied after we land this
patch here. Well maybe should have the same check in gem mmap code to
make sure no driver

> I think it would be better that instead of having special flags in the
> ptes and vmas that you can't follow them to a page structure we would
> add something to the page indicating that you can't grab a reference to
> it. But this might break some use cases as well.

Afaik the problem with that is that there's no free page bits left for
these debug checks. Plus the pte+vma flags are the flags to make this
clear already.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch