Changing vma->vm_file in dma_buf_mmap()

[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?
 
> 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?

Jason