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

["Christian König" <christian.koenig@amd.com>]

Am 22.06.20 um 16:32 schrieb Andrey Grodzovsky:
>
> On 6/22/20 9:18 AM, Christian König wrote:
>> Am 21.06.20 um 08:03 schrieb Andrey Grodzovsky:
>>> 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;
>>> +    }
>>> +
>>
>> While the per file case still looks acceptable this is a clear NAK 
>> since it will massively increase the memory needed for a prime 
>> exported object.
>>
>> I think that this is quite overkill in the first place and for the 
>> hot unplug case we can just use the global dummy page as well.
>>
>> Christian.
>
>
> Global dummy page is good for read access, what do you do on write 
> access ? My first approach was indeed to map at first global dummy 
> page as read only and mark the vma->vm_flags as !VM_SHARED assuming 
> that this would trigger Copy On Write flow in core mm 
> (https://elixir.bootlin.com/linux/v5.7-rc7/source/mm/memory.c#L3977) 
> on the next page fault to same address triggered by a write access but 
> then i realized a new COW page will be allocated for each such mapping 
> and this is much more wasteful then having a dedicated page per GEM 
> object. 

Yeah, but this is only for a very very small corner cases. What we need 
to prevent is increasing the memory usage during normal operation to much.

Using memory during the unplug is completely unproblematic because we 
just released quite a bunch of it by releasing all those system memory 
buffers.

And I'm pretty sure that COWed pages are correctly accounted towards the 
used memory of a process.

So I think if that approach works as intended and the COW pages are 
released again on unmapping it would be the perfect solution to the problem.

Daniel what do you think?

Regards,
Christian.

> We can indeed optimize by allocating this dummy page on the first page 
> fault after device disconnect instead on GEM object creation.
>
> Andrey
>
>
>>
>>> 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;
>>>   +    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;
>>>   };
>>>     /**
>>

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