Re: [PATCH 8/9] drm/ttm: Don't count pages in SG BOs against pages_limit

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

Am 14.04.21 um 11:15 schrieb Daniel Vetter:
> On Wed, Apr 14, 2021 at 08:51:51AM +0200, Christian König wrote:
>> Am 14.04.21 um 08:48 schrieb Felix Kuehling:
>>> Pages in SG BOs were not allocated by TTM. So don't count them against
>>> TTM's pages limit.
>>>
>>> Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
>> Reviewed-by: Christian König <christian.koenig@amd.com>
>>
>> Going to pick that one up for inclusion in drm-misc-next.
> See my other email, but why do we need this? A bit more explanation is imo
> needed here at least, since we still need to guarantee that allocations
> don't over the limit in total for all gpu buffers together. At least until
> the shrinker has landed.
>
> And this here just opens up the barn door without any explanation why it's
> ok.

The SG based BOs might not even be backed by pages. E.g. exported VRAM.

So either they are exported by a driver which should have accounted for 
the allocation, exported by TTM which already did the accounting or 
doesn't even point to pages at all.

This is really a bug fix to recreate the behavior we had before moving 
the accounting to this place.

Christian.

> -Daniel
>
>> Regards,
>> Christian.
>>
>>> ---
>>>    drivers/gpu/drm/ttm/ttm_tt.c | 27 ++++++++++++++++++---------
>>>    1 file changed, 18 insertions(+), 9 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/ttm/ttm_tt.c b/drivers/gpu/drm/ttm/ttm_tt.c
>>> index 5d8820725b75..e8b8c3257392 100644
>>> --- a/drivers/gpu/drm/ttm/ttm_tt.c
>>> +++ b/drivers/gpu/drm/ttm/ttm_tt.c
>>> @@ -317,9 +317,12 @@ int ttm_tt_populate(struct ttm_device *bdev,
>>>    	if (ttm_tt_is_populated(ttm))
>>>    		return 0;
>>> -	atomic_long_add(ttm->num_pages, &ttm_pages_allocated);
>>> -	if (bdev->pool.use_dma32)
>>> -		atomic_long_add(ttm->num_pages, &ttm_dma32_pages_allocated);
>>> +	if (!(ttm->page_flags & TTM_PAGE_FLAG_SG)) {
>>> +		atomic_long_add(ttm->num_pages, &ttm_pages_allocated);
>>> +		if (bdev->pool.use_dma32)
>>> +			atomic_long_add(ttm->num_pages,
>>> +					&ttm_dma32_pages_allocated);
>>> +	}
>>>    	while (atomic_long_read(&ttm_pages_allocated) > ttm_pages_limit ||
>>>    	       atomic_long_read(&ttm_dma32_pages_allocated) >
>>> @@ -350,9 +353,12 @@ int ttm_tt_populate(struct ttm_device *bdev,
>>>    	return 0;
>>>    error:
>>> -	atomic_long_sub(ttm->num_pages, &ttm_pages_allocated);
>>> -	if (bdev->pool.use_dma32)
>>> -		atomic_long_sub(ttm->num_pages, &ttm_dma32_pages_allocated);
>>> +	if (!(ttm->page_flags & TTM_PAGE_FLAG_SG)) {
>>> +		atomic_long_sub(ttm->num_pages, &ttm_pages_allocated);
>>> +		if (bdev->pool.use_dma32)
>>> +			atomic_long_sub(ttm->num_pages,
>>> +					&ttm_dma32_pages_allocated);
>>> +	}
>>>    	return ret;
>>>    }
>>>    EXPORT_SYMBOL(ttm_tt_populate);
>>> @@ -382,9 +388,12 @@ void ttm_tt_unpopulate(struct ttm_device *bdev, struct ttm_tt *ttm)
>>>    	else
>>>    		ttm_pool_free(&bdev->pool, ttm);
>>> -	atomic_long_sub(ttm->num_pages, &ttm_pages_allocated);
>>> -	if (bdev->pool.use_dma32)
>>> -		atomic_long_sub(ttm->num_pages, &ttm_dma32_pages_allocated);
>>> +	if (!(ttm->page_flags & TTM_PAGE_FLAG_SG)) {
>>> +		atomic_long_sub(ttm->num_pages, &ttm_pages_allocated);
>>> +		if (bdev->pool.use_dma32)
>>> +			atomic_long_sub(ttm->num_pages,
>>> +					&ttm_dma32_pages_allocated);
>>> +	}
>>>    	ttm->page_flags &= ~TTM_PAGE_FLAG_PRIV_POPULATED;
>>>    }
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