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

[Daniel Vetter <daniel@ffwll.ch>]

On Tue, Nov 12, 2019 at 10:26:44AM +0100, Thomas Zimmermann wrote:
> Hi
> 
> Am 08.11.19 um 17:27 schrieb Daniel Vetter:
> > 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.
> 
> VRAM helpers use drm_gem_ttm_mmap(), which wraps ttm_bo_mmap_obj().
> These changes should be transparent.

There's still the issue that for dma-buf mmap vs drm mmap you use
different f_mapping, which means ttm's pte shootdown won't work correctly
for dma-buf mmaps. But yeah normal operation for ttm/vram helpers should
be fine.
-Daniel

> 
> Best regards
> Thomas
> 
> > -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
> > 
> 
> -- 
> Thomas Zimmermann
> Graphics Driver Developer
> SUSE Software Solutions Germany GmbH
> Maxfeldstr. 5, 90409 Nürnberg, Germany
> (HRB 36809, AG Nürnberg)
> Geschäftsführer: Felix Imendörffer
> 




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