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

Christian König
2021-02-25T10:33:51Z

Am 24.02.21 um 10:31 schrieb Daniel Vetter:
> On Wed, Feb 24, 2021 at 10:16 AM Thomas Hellström (Intel)
> <thomas_os@shipmail.org> wrote:
>>
>> On 2/24/21 9:45 AM, Daniel Vetter wrote:
>>> 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?
>> Perhaps returning -EINVAL on is_cow_mapping() at mmap time. Either that
>> or allowing MIXEDMAP.
>>
>>>> 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.
>> IIRC the page cache was implemented because of the slowness of the
>> caching mode transition itself, more specifically the wbinvd() call +
>> global TLB flush.

Yes, exactly that. The global TLB flush is what really breaks our neck
here from a performance perspective.

>>> 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.
>> Yes, I guess that was the issue that the comment describes, but the
>> issue wasn't there with vm_insert_mixed() + VM_MIXEDMAP.
>>
>>> Also TTM uses VM_PFNMAP right now for everything, so it can't be a
>>> problem that hurts much :-)
>> Hmm, both 5.11 and drm-tip appears to still use MIXEDMAP?
>>
>> https://elixir.bootlin.com/linux/latest/source/drivers/gpu/drm/ttm/ttm_bo_vm.c#L554
> Uh that's bad, because mixed maps pointing at struct page wont stop
> gup. At least afaik.

Hui? I'm pretty sure MIXEDMAP stops gup as well. Otherwise we would have
already seen tons of problems with the page cache.

Regards,
Christian.

> Christian, do we need to patch this up, and maybe fix up ttm fault
> handler to use io_mapping so the vm_insert_pfn stuff is fast?
> -Daniel