[Intel-gfx] [PATCH v3 3/3] drm/vgem: Keep a reference to the dmabuf when mmaped

[Chris Wilson]

On Wed, Oct 05, 2016 at 03:11:00PM +0200, Daniel Vetter wrote:
> On Fri, Sep 30, 2016 at 02:59:59PM +0100, Chris Wilson wrote:
> > In order to keep the dmabuf alive whilst the mmap is, we need to hold a
> > reference to the dmabuf and not the backing object. This is important as
> > the dmabuf not only keeps the object alive, but also the device so that
> > 
> > 	dmabuf = vgem_create_dmabuf();
> > 	ptr = mmap(... dmabuf ...);
> > 	close(dmabuf);
> > 
> > persists across module-unload as well as device closure.
> 
> I don't see where we grab the ref to the dma-buf here instead of the
> backing storage. And doesn't the exact same issue happen when you use dumb
> mmap? Or maybe I'm just a bit confused about what's going on here ...

The reference to the dmabuf is in vma->vm_file, which was used to keep
the module alive. So this might be overzealous, in that there shouldn't
be a way to get back from the shmemfs object to the module. However,
that does make me aware of a failure path we have in patches that do add
callbacks from shmemfs to the driver...

At least using the ptr after closing the module is ok. So this patch is
not required.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre