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

Thomas Hellström (Intel)
2021-02-26T09:43:25Z

On 2/25/21 4:49 PM, Daniel Vetter wrote:
> On Thu, Feb 25, 2021 at 11:44 AM Daniel Vetter <daniel@ffwll.ch> wrote:
>> On Thu, Feb 25, 2021 at 11:28:31AM +0100, Christian König wrote:
>>> 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.
>> On any architecture which has CONFIG_ARCH_HAS_PTE_SPECIAL vm_insert_mixed
>> boils down to vm_insert_pfn wrt gup. And special pte stops gup fast path.
>>
>> But if you don't have VM_IO or VM_PFNMAP set, then I'm not seeing how
>> you're stopping gup slow path. See check_vma_flags() in mm/gup.c.
>>
>> Also if you don't have CONFIG_ARCH_HAS_PTE_SPECIAL then I don't think
>> vm_insert_mixed even works on iomem pfns. There's the devmap exception,
>> but we're not devmap. Worse ttm abuses some accidental codepath to smuggle
>> in hugepte support by intentionally not being devmap.
>>
>> So I'm really not sure this works as we think it should. Maybe good to do
>> a quick test program on amdgpu with a buffer in system memory only and try
>> to do direct io into it. If it works, you have a problem, and a bad one.
> That's probably impossible, since a quick git grep shows that pretty
> much anything reasonable has special ptes: arc, arm, arm64, powerpc,
> riscv, s390, sh, sparc, x86. I don't think you'll have a platform
> where you can plug an amdgpu in and actually exercise the bug :-)

Hm. AFAIK _insert_mixed() doesn't set PTE_SPECIAL on system pages, so I
don't see what should be stopping gup to those?

/Thomas

>
> So maybe we should just switch over to VM_PFNMAP for ttm for more clarity?
> -Daniel
>
>
>>> 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
>> --
>> Daniel Vetter
>> Software Engineer, Intel Corporation
>> http://blog.ffwll.ch
>
>