Re: Changing vma->vm_file in dma_buf_mmap()

From: Daniel Vetter
Date: Wed Sep 16 2020 - 17:08:51 EST

On Wed, Sep 16, 2020 at 4:14 PM Christian König
<christian.koenig@xxxxxxx> 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.

> >> 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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Daniel Vetter
Software Engineer, Intel Corporation