[Linaro-mm-sig] Re: [PATCH v6 02/22] drm/gem: Move mapping of imported dma-bufs to drm_gem_mmap_obj()

[Dmitry Osipenko]

On 7/4/22 15:33, Christian König wrote:
> Am 30.06.22 um 01:06 schrieb Dmitry Osipenko:
>> On 6/29/22 11:43, Thomas Hellström (Intel) wrote:
>>> On 6/29/22 10:22, Dmitry Osipenko wrote:
>>>> On 6/29/22 09:40, Thomas Hellström (Intel) wrote:
>>>>> On 5/27/22 01:50, Dmitry Osipenko wrote:
>>>>>> Drivers that use drm_gem_mmap() and drm_gem_mmap_obj() helpers don't
>>>>>> handle imported dma-bufs properly, which results in mapping of
>>>>>> something
>>>>>> else than the imported dma-buf. For example, on NVIDIA Tegra we get a
>>>>>> hard
>>>>>> lockup when userspace writes to the memory mapping of a dma-buf that
>>>>>> was
>>>>>> imported into Tegra's DRM GEM.
>>>>>>
>>>>>> To fix this bug, move mapping of imported dma-bufs to
>>>>>> drm_gem_mmap_obj().
>>>>>> Now mmaping of imported dma-bufs works properly for all DRM drivers.
>>>>> Same comment about Fixes: as in patch 1,
>>>>>> Cc: stable at vger.kernel.org
>>>>>> Signed-off-by: Dmitry Osipenko <dmitry.osipenko at collabora.com>
>>>>>> ---
>>>>>>     drivers/gpu/drm/drm_gem.c              | 3 +++
>>>>>>     drivers/gpu/drm/drm_gem_shmem_helper.c | 9 ---------
>>>>>>     drivers/gpu/drm/tegra/gem.c            | 4 ++++
>>>>>>     3 files changed, 7 insertions(+), 9 deletions(-)
>>>>>>
>>>>>> diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c
>>>>>> index 86d670c71286..7c0b025508e4 100644
>>>>>> --- a/drivers/gpu/drm/drm_gem.c
>>>>>> +++ b/drivers/gpu/drm/drm_gem.c
>>>>>> @@ -1038,6 +1038,9 @@ int drm_gem_mmap_obj(struct drm_gem_object
>>>>>> *obj,
>>>>>> unsigned long obj_size,
>>>>>>         if (obj_size < vma->vm_end - vma->vm_start)
>>>>>>             return -EINVAL;
>>>>>>     +    if (obj->import_attach)
>>>>>> +        return dma_buf_mmap(obj->dma_buf, vma, 0);
>>>>> If we start enabling mmaping of imported dma-bufs on a majority of
>>>>> drivers in this way, how do we ensure that user-space is not blindly
>>>>> using the object mmap without calling the needed DMA_BUF_IOCTL_SYNC
>>>>> which is needed before and after cpu access of mmap'ed dma-bufs?
>>>>>
>>>>> I was under the impression (admittedly without looking) that the few
>>>>> drivers that actually called into dma_buf_mmap() had some private
>>>>> user-mode driver code in place that ensured this happened.
>>>> Since it's a userspace who does the mapping, then it should be a
>>>> responsibility of userspace to do all the necessary syncing.
>>> Sure, but nothing prohibits user-space to ignore the syncing thinking
>>> "It works anyway", testing those drivers where the syncing is a NOP. And
>>> when a driver that finally needs syncing is tested it's too late to fix
>>> all broken user-space.
>>>
>>>>    I'm not
>>>> sure whether anyone in userspace really needs to map imported dma-bufs
>>>> in practice. Nevertheless, this use-case is broken and should be fixed
>>>> by either allowing to do the mapping or prohibiting it.
>>>>
>>> Then I'd vote for prohibiting it, at least for now. And for the future
>>> moving forward we could perhaps revisit the dma-buf need for syncing,
>>> requiring those drivers that actually need it to implement emulated
>>> coherent memory which can be done not too inefficiently (vmwgfx being
>>> one example).
>> Alright, I'll change it to prohibit the mapping. This indeed should be a
>> better option.
> 
> Oh, yes please. But I would expect that some people start screaming.
> 
> Over time I've got tons of TTM patches because people illegally tried to
> mmap() imported DMA-bufs in their driver.
> 
> Anyway this is probably the right thing to do and we can work on fixing
> the fallout later on.

I already sent out the patch [1] that prohibits the mapping. Would be
great if you all could take a look and give a r-b, thanks in advance.

[1] https://patchwork.freedesktop.org/patch/492148/

-- 
Best regards,
Dmitry