dma-buf: Require VM_PFNMAP vma for mmap

tldr; DMA buffers aren't normal memory, expecting that you can use
them like that (like calling get_user_pages works, or that they're
accounting like any other normal memory) cannot be guaranteed.

Since some userspace only runs on integrated devices, where all
buffers are actually all resident system memory, there's a huge
temptation to assume that a struct page is always present and useable
like for any more pagecache backed mmap. This has the potential to
result in a uapi nightmare.

To stop this gap require that DMA buffer mmaps are VM_PFNMAP, which
blocks get_user_pages and all the other struct page based
infrastructure for everyone. In spirit this is the uapi counterpart to
the kernel-internal CONFIG_DMABUF_DEBUG.

Motivated by a recent patch which wanted to swich the system dma-buf
heap to vm_insert_page instead of vm_insert_pfn.

v2:

Jason brought up that we also want to guarantee that all ptes have the
pte_special flag set, to catch fast get_user_pages (on architectures
that support this). Allowing VM_MIXEDMAP (like VM_SPECIAL does) would
still allow vm_insert_page, but limiting to VM_PFNMAP will catch that.

From auditing the various functions to insert pfn pte entires
(vm_insert_pfn_prot, remap_pfn_range and all it's callers like
dma_mmap_wc) it looks like VM_PFNMAP is already required anyway, so
this should be the correct flag to check for.

v3: Change to WARN_ON_ONCE (Thomas Zimmermann)

References: https://lore.kernel.org/lkml/CAKMK7uHi+mG0z0HUmNt13QCCvutuRVjpcR0NjRL12k-WbWzkRg@mail.gmail.com/
Acked-by: Christian König <christian.koenig@amd.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: John Stultz <john.stultz@linaro.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: linux-media@vger.kernel.org
Cc: linaro-mm-sig@lists.linaro.org
--
Ok I entirely forgot about this patch but stumbled over it and checked
what's up with it no. I think it's ready now for merging:
- shmem helper patches to fix up vgem landed
- ttm has been fixed since a while
- I don't think we've had any other open issues

Time to lock down this uapi contract for real?
-Daniel

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