Re: [Linaro-mm-sig] Changing vma->vm_file in dma_buf_mmap()

Christian König
2020-09-16T19:09:36Z

Am 16.09.20 um 17:24 schrieb Daniel Vetter:
> On Wed, Sep 16, 2020 at 4:14 PM Christian König
> <christian.koenig@amd.com> wrote:
>> Am 16.09.20 um 16:07 schrieb Jason Gunthorpe:
>>> On Wed, Sep 16, 2020 at 11:53:59AM +0200, Daniel Vetter wrote:
>>>
>>>> But within the driver, we generally need thousands of these, and that
>>>> tends to bring fd exhaustion problems with it. That's why all the private
>>>> buffer objects which aren't shared with other process or other drivers are
>>>> handles only valid for a specific fd instance of the drm chardev (each
>>>> open gets their own namespace), and only for ioctls done on that chardev.
>>>> And for mmap we assign fake (but unique across all open fd on it) offsets
>>>> within the overall chardev. Hence all the pgoff mangling and re-mangling.
>>> Are they still unique struct files? Just without a fdno?
>> Yes, exactly.
> Not entirely, since dma-buf happened after drm chardev, so for that
> historical reason the underlying struct file is shared, since it's the
> drm chardev. But since that's per-device we don't have a problem in
> practice with different vm_ops, since those are also per-device. But
> yeah we could fish out some entirely hidden per-object struct file if
> that's required for some mm internal reasons.

Hui? Ok that is just the handling in i915, isn't it?

As far as I know we create an unique struct file for each DMA-buf.

Regards,
Christian.

> -Daniel
>
>>>> Hence why we'd like to be able to forward aliasing mappings and adjust the
>>>> file and pgoff, while hopefully everything keeps working. I thought this
>>>> would work, but Christian noticed it doesn't really.
>>> It seems reasonable to me that the dma buf should be the owner of the
>>> VMA, otherwise like you say, there is a big mess attaching the custom
>>> vma ops and what not to the proper dma buf.
>>>
>>> I don't see anything obviously against this in mmap_region() - why did
>>> Chritian notice it doesn't really work?
>> To clarify I think this might work.
>>
>> I just had the same "Is that legal?", "What about security?", etc..
>> questions you raised as well.
>>
>> It seems like a source of trouble so I thought better ask somebody more
>> familiar with that.
>>
>> Christian.
>>
>>> Jason
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