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

[Thomas Zimmermann]

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 at redhat.com>
>>> Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
>>> Signed-off-by: Rob Herring <robh at 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.

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 at 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

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