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

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

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. 
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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