Re: [PATCH v2 1/8] drm: Add dummy page per device or GEM object

[Andrey Grodzovsky <Andrey.Grodzovsky@amd.com>]

On 6/22/20 5:35 AM, Daniel Vetter wrote:
> On Sun, Jun 21, 2020 at 02:03:01AM -0400, Andrey Grodzovsky wrote:
>> Will be used to reroute CPU mapped BO's page faults once
>> device is removed.
>>
>> Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
>> ---
>>   drivers/gpu/drm/drm_file.c  |  8 ++++++++
>>   drivers/gpu/drm/drm_prime.c | 10 ++++++++++
>>   include/drm/drm_file.h      |  2 ++
>>   include/drm/drm_gem.h       |  2 ++
>>   4 files changed, 22 insertions(+)
>>
>> diff --git a/drivers/gpu/drm/drm_file.c b/drivers/gpu/drm/drm_file.c
>> index c4c704e..67c0770 100644
>> --- a/drivers/gpu/drm/drm_file.c
>> +++ b/drivers/gpu/drm/drm_file.c
>> @@ -188,6 +188,12 @@ struct drm_file *drm_file_alloc(struct drm_minor *minor)
>>   			goto out_prime_destroy;
>>   	}
>>   
>> +	file->dummy_page = alloc_page(GFP_KERNEL | __GFP_ZERO);
>> +	if (!file->dummy_page) {
>> +		ret = -ENOMEM;
>> +		goto out_prime_destroy;
>> +	}
>> +
>>   	return file;
>>   
>>   out_prime_destroy:
>> @@ -284,6 +290,8 @@ void drm_file_free(struct drm_file *file)
>>   	if (dev->driver->postclose)
>>   		dev->driver->postclose(dev, file);
>>   
>> +	__free_page(file->dummy_page);
>> +
>>   	drm_prime_destroy_file_private(&file->prime);
>>   
>>   	WARN_ON(!list_empty(&file->event_list));
>> diff --git a/drivers/gpu/drm/drm_prime.c b/drivers/gpu/drm/drm_prime.c
>> index 1de2cde..c482e9c 100644
>> --- a/drivers/gpu/drm/drm_prime.c
>> +++ b/drivers/gpu/drm/drm_prime.c
>> @@ -335,6 +335,13 @@ int drm_gem_prime_fd_to_handle(struct drm_device *dev,
>>   
>>   	ret = drm_prime_add_buf_handle(&file_priv->prime,
>>   			dma_buf, *handle);
>> +
>> +	if (!ret) {
>> +		obj->dummy_page = alloc_page(GFP_KERNEL | __GFP_ZERO);
>> +		if (!obj->dummy_page)
>> +			ret = -ENOMEM;
>> +	}
>> +
>>   	mutex_unlock(&file_priv->prime.lock);
>>   	if (ret)
>>   		goto fail;
>> @@ -1006,6 +1013,9 @@ void drm_prime_gem_destroy(struct drm_gem_object *obj, struct sg_table *sg)
>>   		dma_buf_unmap_attachment(attach, sg, DMA_BIDIRECTIONAL);
>>   	dma_buf = attach->dmabuf;
>>   	dma_buf_detach(attach->dmabuf, attach);
>> +
>> +	__free_page(obj->dummy_page);
>> +
>>   	/* remove the reference */
>>   	dma_buf_put(dma_buf);
>>   }
>> diff --git a/include/drm/drm_file.h b/include/drm/drm_file.h
>> index 19df802..349a658 100644
>> --- a/include/drm/drm_file.h
>> +++ b/include/drm/drm_file.h
>> @@ -335,6 +335,8 @@ struct drm_file {
>>   	 */
>>   	struct drm_prime_file_private prime;
>>   
> Kerneldoc for these please, including why we need them and when. E.g. the
> one in gem_bo should say it's only for exported buffers, so that we're not
> colliding security spaces.
>
>> +	struct page *dummy_page;
>> +
>>   	/* private: */
>>   #if IS_ENABLED(CONFIG_DRM_LEGACY)
>>   	unsigned long lock_count; /* DRI1 legacy lock count */
>> diff --git a/include/drm/drm_gem.h b/include/drm/drm_gem.h
>> index 0b37506..47460d1 100644
>> --- a/include/drm/drm_gem.h
>> +++ b/include/drm/drm_gem.h
>> @@ -310,6 +310,8 @@ struct drm_gem_object {
>>   	 *
>>   	 */
>>   	const struct drm_gem_object_funcs *funcs;
>> +
>> +	struct page *dummy_page;
>>   };
> I think amdgpu doesn't care, but everyone else still might care somewhat
> about flink. That also shares buffers, so also needs to allocate the
> per-bo dummy page.


Hi, back to this topic after a long context switch for internal project.

I don't see why for FLINK we can't use same dummy page from struct 
drm_gem_object - looking
at drm_gem_flink_ioctl I see that the underlying object we look up is still of 
type drm_gem_object.
Why we need per BO (TTM BO  I assume?) dummy page for this ?

Andrey


>
> I also wonder whether we shouldn't have a helper to look up the dummy
> page, just to encode in core code how it's supposedo to cascade.
> -Daniel
>
>>   
>>   /**
>> -- 
>> 2.7.4
>>
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