Re: [PATCH v2] drm/gem: Fix mmap fake offset handling for drm_gem_object_funcs.mmap

[Daniel Vetter <daniel@ffwll.ch>]

On Fri, Nov 08, 2019 at 05:27:59PM +0100, Daniel Vetter wrote:
> On Fri, Oct 25, 2019 at 09:30:42AM +0200, Daniel Vetter wrote:
> > On Thu, Oct 24, 2019 at 02:18:59PM -0500, Rob Herring wrote:
> > > Commit c40069cb7bd6 ("drm: add mmap() to drm_gem_object_funcs")
> > > introduced a GEM object mmap() hook which is expected to subtract the
> > > fake offset from vm_pgoff. However, for mmap() on dmabufs, there is not
> > > a fake offset.
> > > 
> > > To fix this, let's always call mmap() object callback with an offset of 0,
> > > and leave it up to drm_gem_mmap_obj() to remove the fake offset.
> > > 
> > > TTM still needs the fake offset, so we have to add it back until that's
> > > fixed.
> > > 
> > > Fixes: c40069cb7bd6 ("drm: add mmap() to drm_gem_object_funcs")
> > > Cc: Gerd Hoffmann <kraxel@redhat.com>
> > > Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
> > > Signed-off-by: Rob Herring <robh@kernel.org>
> > > ---
> > > v2:
> > > - Move subtracting the fake offset out of mmap() obj callbacks.
> > > 
> > > I've tested shmem, but not ttm. Hopefully, I understood what's needed 
> > > for TTM.
> 
> So unfortunately I'm already having regrets on this. We might even have
> broken some of the ttm drivers here.
> 
> Trouble is, if you need to shoot down userspace ptes of a bo (because it's
> getting moved or whatever), then we do that with unmap_mapping_range.
> Which means each bo needs to be mapping with a unique (offset,
> adress_space), or it won't work. By remapping all the bo to 0 we've broken
> this. We've also had this broken this for a while for the simplistic
> dma-buf mmap, since without any further action we'll reuse the address
> space of the dma-buf inode, not of the drm_device.
> 
> Strangely both etnaviv and msm have a comment to that effect - grep for
> unmap_mapping_range. But neither actually uses it.
> 
> Not exactly sure what's the best course of action here now.
> 
> Also adding Thomas Zimmermann, who's worked on all the vram helpers.

Correction, I missed the unmap_mapping_range in the vma node manager
header, so didn't spot the drivers using drm_vma_node_unmap. We did break
all the ttm stuff :-/

Plus panfrost, which is using drm_gem_shmem_purge_locked.
-Daniel

> -Daniel
> 
> > > 
> > > Rob
> > > 
> > >  drivers/gpu/drm/drm_gem.c              | 3 +++
> > >  drivers/gpu/drm/drm_gem_shmem_helper.c | 3 ---
> > >  drivers/gpu/drm/ttm/ttm_bo_vm.c        | 7 +++++++
> > >  include/drm/drm_gem.h                  | 4 +++-
> > >  4 files changed, 13 insertions(+), 4 deletions(-)
> > > 
> > > diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c
> > > index 56f42e0f2584..2f2b889096b0 100644
> > > --- a/drivers/gpu/drm/drm_gem.c
> > > +++ b/drivers/gpu/drm/drm_gem.c
> > > @@ -1106,6 +1106,9 @@ int drm_gem_mmap_obj(struct drm_gem_object *obj, unsigned long obj_size,
> > >  		return -EINVAL;
> > >  
> > >  	if (obj->funcs && obj->funcs->mmap) {
> > > +		/* Remove the fake offset */
> > > +		vma->vm_pgoff -= drm_vma_node_start(&obj->vma_node);
> > > +
> > >  		ret = obj->funcs->mmap(obj, vma);
> > >  		if (ret)
> > >  			return ret;
> > > diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
> > > index a878c787b867..e8061c64c480 100644
> > > --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
> > > +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
> > > @@ -542,9 +542,6 @@ int drm_gem_shmem_mmap(struct drm_gem_object *obj, struct vm_area_struct *vma)
> > >  	vma->vm_page_prot = pgprot_decrypted(vma->vm_page_prot);
> > >  	vma->vm_ops = &drm_gem_shmem_vm_ops;
> > >  
> > > -	/* Remove the fake offset */
> > > -	vma->vm_pgoff -= drm_vma_node_start(&shmem->base.vma_node);
> > > -
> > >  	return 0;
> > >  }
> > >  EXPORT_SYMBOL_GPL(drm_gem_shmem_mmap);
> > > diff --git a/drivers/gpu/drm/ttm/ttm_bo_vm.c b/drivers/gpu/drm/ttm/ttm_bo_vm.c
> > > index 1a9db691f954..08902c7290a5 100644
> > > --- a/drivers/gpu/drm/ttm/ttm_bo_vm.c
> > > +++ b/drivers/gpu/drm/ttm/ttm_bo_vm.c
> > > @@ -482,6 +482,13 @@ EXPORT_SYMBOL(ttm_bo_mmap);
> > >  int ttm_bo_mmap_obj(struct vm_area_struct *vma, struct ttm_buffer_object *bo)
> > >  {
> > >  	ttm_bo_get(bo);
> > > +
> > > +	/*
> > > +	 * FIXME: &drm_gem_object_funcs.mmap is called with the fake offset
> > > +	 * removed. Add it back here until the rest of TTM works without it.
> > > +	 */
> > > +	vma->vm_pgoff += drm_vma_node_start(&bo->base.vma_node);
> > > +
> > >  	ttm_bo_mmap_vma_setup(bo, vma);
> > >  	return 0;
> > >  }
> > > diff --git a/include/drm/drm_gem.h b/include/drm/drm_gem.h
> > > index e71f75a2ab57..c56cbb3509e0 100644
> > > --- a/include/drm/drm_gem.h
> > > +++ b/include/drm/drm_gem.h
> > > @@ -159,7 +159,9 @@ struct drm_gem_object_funcs {
> > >  	 *
> > >  	 * The callback is used by by both drm_gem_mmap_obj() and
> > >  	 * drm_gem_prime_mmap().  When @mmap is present @vm_ops is not
> > > -	 * used, the @mmap callback must set vma->vm_ops instead.
> > > +	 * used, the @mmap callback must set vma->vm_ops instead. The @mmap
> > > +	 * callback is always called with a 0 offset. The caller will remove
> > > +	 * the fake offset as necessary.
> > >  	 *
> > 
> > Maybe remove this empty comment line here while at it. With that
> > 
> > Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> > 
> > I think I'll follow up with a patch to annotate drm_gem_mmap_obj as
> > deprecated and that instead this here should be used.
> > -Daniel
> > 
> > >  	 */
> > >  	int (*mmap)(struct drm_gem_object *obj, struct vm_area_struct *vma);
> > > -- 
> > > 2.20.1
> > > 
> > 
> > -- 
> > Daniel Vetter
> > Software Engineer, Intel Corporation
> > http://blog.ffwll.ch
> 
> -- 
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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