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

Daniel Vetter
2020-09-17T06:23:37Z

On Wed, Sep 16, 2020 at 5:31 PM Christian König
<ckoenig.leichtzumerken@gmail.com> wrote:
>
> 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.

Yes dma-buf, but that gets forwarded to the original drm chardev which
originally exported the buffer. It's only there where the forwarding
chain stops. The other thing is that iirc we have a singleton
anon_inode behind all the dma-buf, so they'd share all the same
address_space and so would all alias for unmap_mapping_range (I think
at least).
-Daniel

>
> 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
> >> _______________________________________________
> >> dri-devel mailing list
> >> dri-devel@lists.freedesktop.org
> >> https://lists.freedesktop.org/mailman/listinfo/dri-devel
> >
> >
>

--
Daniel Vetter
Software Engineer, Intel Corporation