[PATCH] arm64/dma-mapping: fix DMA_ATTR_FORCE_CONTIGUOUS mmaping code

[Andrzej Hajda]

On 30.03.2017 09:40, Geert Uytterhoeven wrote:
> Hi Andrzej,
>
> On Thu, Mar 30, 2017 at 8:51 AM, Andrzej Hajda <a.hajda at samsung.com> wrote:
>> On 29.03.2017 17:33, Robin Murphy wrote:
>>> On 29/03/17 16:12, Andrzej Hajda wrote:
>>>> On 29.03.2017 14:55, Robin Murphy wrote:
>>>>> On 29/03/17 11:05, Andrzej Hajda wrote:
>>>>>> In case of DMA_ATTR_FORCE_CONTIGUOUS allocations vm_area->pages
>>>>>> is invalid. __iommu_mmap_attrs and __iommu_get_sgtable cannot use
>>>>>> it. In first case temporary pages array is passed to iommu_dma_mmap,
>>>>>> in 2nd case single entry sg table is created directly instead
>>>>>> of calling helper.
>>>>>>
>>>>>> Fixes: 44176bb ("arm64: Add support for DMA_ATTR_FORCE_CONTIGUOUS to IOMMU")
>>>>>> Signed-off-by: Andrzej Hajda <a.hajda at samsung.com>
>>>>>> ---
>>>>>> Hi,
>>>>>>
>>>>>> I am not familiar with this framework so please don't be too cruel ;)
>>>>>> Alternative solution I see is to always create vm_area->pages,
>>>>>> I do not know which approach is preferred.
>>>>>>
>>>>>> Regards
>>>>>> Andrzej
>>>>>> ---
>>>>>>  arch/arm64/mm/dma-mapping.c | 40 ++++++++++++++++++++++++++++++++++++++--
>>>>>>  1 file changed, 38 insertions(+), 2 deletions(-)
>>>>>>
>>>>>> diff --git a/arch/arm64/mm/dma-mapping.c b/arch/arm64/mm/dma-mapping.c
>>>>>> index f7b5401..bba2bc8 100644
>>>>>> --- a/arch/arm64/mm/dma-mapping.c
>>>>>> +++ b/arch/arm64/mm/dma-mapping.c
>>>>>> @@ -704,7 +704,30 @@ static int __iommu_mmap_attrs(struct device *dev, struct vm_area_struct *vma,
>>>>>>            return ret;
>>>>>>
>>>>>>    area = find_vm_area(cpu_addr);
>>>>>> -  if (WARN_ON(!area || !area->pages))
>>>>>> +  if (WARN_ON(!area))
>>>>> >From the look of things, it doesn't seem strictly necessary to change
>>>>> this, but whether that's a good thing is another matter. I'm not sure
>>>>> that dma_common_contiguous_remap() should really be leaving a dangling
>>>>> pointer in area->pages as it apparently does... :/
>>>>>
>>>>>> +          return -ENXIO;
>>>>>> +
>>>>>> +  if (attrs & DMA_ATTR_FORCE_CONTIGUOUS) {
>>>>>> +          struct page *page = vmalloc_to_page(cpu_addr);
>>>>>> +          unsigned int count = size >> PAGE_SHIFT;
>>>>>> +          struct page **pages;
>>>>>> +          unsigned long pfn;
>>>>>> +          int i;
>>>>>> +
>>>>>> +          pages = kmalloc_array(count, sizeof(*pages), GFP_KERNEL);
>>>>>> +          if (!pages)
>>>>>> +                  return -ENOMEM;
>>>>>> +
>>>>>> +          for (i = 0, pfn = page_to_pfn(page); i < count; i++)
>>>>>> +                  pages[i] = pfn_to_page(pfn + i);
>>>>>> +
>>>>>> +          ret = iommu_dma_mmap(pages, size, vma);
>>>>> /**
>>>>>  * iommu_dma_mmap - Map a buffer into provided user VMA
>>>>>  * @pages: Array representing buffer from iommu_dma_alloc()
>>>>>    ...
>>>>>
>>>>> In this case, the buffer has not come from iommu_dma_alloc(), so passing
>>>>> into iommu_dma_mmap() is wrong by contract, even if having to fake up a
>>>>> page array wasn't enough of a red flag. Given that a FORCE_CONTIGUOUS
>>>>> allocation is essentially the same as for the non-IOMMU case, I think it
>>>>> should be sufficient to defer to that path, i.e.:
>>>>>
>>>>>     if (attrs & DMA_ATTR_FORCE_CONTIGUOUS)
>>>>>             return __swiotlb_mmap(dev, vma, cpu_addr,
>>>>>                             phys_to_dma(virt_to_phys(cpu_addr)),
>>>>>                             size, attrs);
>>>> Maybe I have make mistake somewhere but it does not work, here and below
>>>> (hangs or crashes). I suppose it can be due to different address
>>>> translations, my patch uses
>>>>     page = vmalloc_to_page(cpu_addr).
>>>> And here we have:
>>>>     handle = phys_to_dma(dev, virt_to_phys(cpu_addr)); // in
>>>> __iommu_mmap_attrs
>>>>     page = phys_to_page(dma_to_phys(dev, handle)); // in
>>>> __swiotlb_get_sgtable
>>>> I guess it is similarly in __swiotlb_mmap.
>>>>
>>>> Are these translations equivalent?
>>> Ah, my mistake, sorry - I managed to forget that cpu_addr is always
>>> remapped for FORCE_CONTIGUOUS (*and* somehow overlook the use of
>>> vmalloc_to_page() in the patch itself), so the virt_to_phys() part of my
>>> example is indeed bogus. The general point still stands, though.
>> I guess Geert's proposition to create pages permanently is also not
>> acceptable[2]. So how to fix crashes which appeared after patch adding
> If I'm not mistaken, creating the pages permanently is what the
> !DMA_ATTR_FORCE_CONTIGUOUS does? The only difference is where
> the underlying memory is allocated from.
>
> Am I missing something?

Quoting Robin from his response:
> in general is is not
> safe to assume a coherent allocation is backed by struct pages at all

As I said before I am not familiar with the subject, so it is possible I
misunderstood something.

Regards
Andrzej


>
> Thanks!
>
>> support to DMA_ATTR_FORCE_CONTIGUOUS in IOMMU on arm64 [1]?
>> Maybe temporary solution is to drop the patch until proper handling of
>> mmapping is proposed, without it the patch looks incomplete and causes
>> regression.
>> Moreover __iommu_alloc_attrs uses also dma_common_contiguous_remap which
>> also assumes existence of struct pages.
>>
>> [1]: https://patchwork.kernel.org/patch/9609551/
>> [2]: https://www.spinics.net/lists/arm-kernel/msg572688.html
> Gr{oetje,eeting}s,
>
>                         Geert
>
> --
> Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert at linux-m68k.org
>
> In personal conversations with technical people, I call myself a hacker. But
> when I'm talking to journalists I just say "programmer" or something like that.
>                                 -- Linus Torvalds
>
>
>