[PATCH] drm/ttm: fix adding foreign BOs to the LRU during init

[Christian König]

It's correct that exported buffers can't be moved to another domain or 
swapped to disk while another device is using it.

But for imports that's a different story:
> an imported object should never end up on a LRU list in TTM because
> TTM wouldn't know how to evict it.
Even currently the imported objects are actually added to the LRU. The 
problem is just that they are not added immediately during creation, but 
only with the first command submission.

E.g. what you can do as userspace process is importing a lot of buffers 
from a different device into the GFX drivers and never use it. This way 
you can force the kernel into running out of GTT address space.

Regards,
Christian.

Am 09.01.2016 um 08:05 schrieb Thomas Hellstrom:
> Hi!
>
> I might be misunderderstanding the use-case here, but IIRC the
> discussion with TTM vs imported / exported buffer objects is that a
> buffer object needs to be marked NO_EVICT in TTM before it's exported
> and an imported object should never end up on a LRU list in TTM because
> TTM wouldn't know how to evict it.
>
> /Thomas
>   
>
> On 01/08/2016 02:41 PM, Christian König wrote:
>> From: Christian König <christian.koenig at amd.com>
>>
>> If we import a BO with an eternal reservation object we don't
>> reserve/unreserve it. So we never add it to the LRU causing a possible
>> deny of service.
>>
>> Signed-off-by: Christian König <christian.koenig at amd.com>
>> ---
>>   drivers/gpu/drm/ttm/ttm_bo.c | 8 +++++++-
>>   1 file changed, 7 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/gpu/drm/ttm/ttm_bo.c b/drivers/gpu/drm/ttm/ttm_bo.c
>> index 745e996..a98a5d5 100644
>> --- a/drivers/gpu/drm/ttm/ttm_bo.c
>> +++ b/drivers/gpu/drm/ttm/ttm_bo.c
>> @@ -1170,9 +1170,15 @@ int ttm_bo_init(struct ttm_bo_device *bdev,
>>   	if (likely(!ret))
>>   		ret = ttm_bo_validate(bo, placement, interruptible, false);
>>   
>> -	if (!resv)
>> +	if (!resv) {
>>   		ttm_bo_unreserve(bo);
>>   
>> +	} else if (!(bo->mem.placement & TTM_PL_FLAG_NO_EVICT)) {
>> +		spin_lock(&bo->glob->lru_lock);
>> +		ttm_bo_add_to_lru(bo);
>> +		spin_unlock(&bo->glob->lru_lock);
>> +	}
>> +
>>   	if (unlikely(ret))
>>   		ttm_bo_unref(&bo);
>>   
>
>