Re: [Linaro-mm-sig] [PATCH 1/2] dma-buf: Require VM_PFNMAP vma for mmap

Thomas Hellström (Intel)
2021-02-24T07:53:26Z

On 2/23/21 11:59 AM, Daniel Vetter wrote:
> 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.
>
If we require VM_PFNMAP, for ordinary page mappings, we also need to
disallow COW mappings, since it will not work on architectures that
don't have CONFIG_ARCH_HAS_PTE_SPECIAL, (see the docs for vm_normal_page()).

Also worth noting is the comment in  ttm_bo_mmap_vma_setup() with
possible performance implications with x86 + PAT + VM_PFNMAP + normal
pages. That's a very old comment, though, and might not be valid anymore.

/Thomas