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

Daniel Vetter
2021-02-24T08:49:03Z

On Wed, Feb 24, 2021 at 8:46 AM Thomas Hellström (Intel)
<thomas_os@shipmail.org> wrote:
>
>
> 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()).

Hm I figured everyone just uses MAP_SHARED for buffer objects since
COW really makes absolutely no sense. How would we enforce this?

> 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.

I think that's why ttm has a page cache for these, because it indeed
sucks. The PAT changes on pages are rather expensive.

There is still an issue for iomem mappings, because the PAT validation
does a linear walk of the resource tree (lol) for every vm_insert_pfn.
But for i915 at least this is fixed by using the io_mapping
infrastructure, which does the PAT reservation only once when you set
up the mapping area at driver load.

Also TTM uses VM_PFNMAP right now for everything, so it can't be a
problem that hurts much :-)
-Daniel
--
Daniel Vetter
Software Engineer, Intel Corporation