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 14:25 schrieb Daniel Vetter:
> On Wed, Apr 14, 2021 at 12:49 PM Christian König
> <ckoenig.leichtzumerken@gmail.com> wrote:
>> Am 14.04.21 um 12:26 schrieb Daniel Vetter:
>>> On Wed, Apr 14, 2021 at 11:19:41AM +0200, Christian König wrote:
>>>> 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.
>>> Throw that into the commit message and a-b: me. Ideally with a Fixes: line
>>> or so pointing at the offending commit that broke stuff. Commit messages
>>> should really go into more detail when there's an entire story behind a
>>> small change like this one.
>> Sorry I though that this would be obvious :)
>>
>> I've already pushed the patch in the morning, but going to keep that in
>> mind for the next time.
> I'll keep reminding you to pls elaborate more in commit messages, it's
> coming up every once in a while :-)

Well, describing stuff in a commit message which is obvious is just 
redundant.

I can of course explain the whole background of the code in question in 
the commit message, but for obvious bug fixes like this it is just overkill.

> Also in general I think a few days of letting patches soak out there,
> especially shared code, is good curtesy. Some folks demand 2 weeks,
> which I think is too much, but less than 24h just means you're
> guaranteed to leave out half the globe with their feedback. Which
> isn't great.

Well for structural changes I certainly agree, but not for a bug fix 
which adds a missing check for a special case.

Christian.

>
> Driver code I don't care since there you know all the stakeholders ofc.
> -Daniel
>
>> Christian.
>>
>>> -Daniel
>>>
>>>> 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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