Re: [PATCH 1/5] drm/vmwgfx: unbind in vmw_ttm_unpopulate

["Christian König" <ckoenig.leichtzumerken@gmail.com>]

Am 23.07.21 um 10:47 schrieb Daniel Vetter:
> On Thu, Jul 22, 2021 at 02:41:23PM +0200, Christian König wrote:
>> Doing this in vmw_ttm_destroy() is to late.
>>
>> It turned out that this is not a good idea at all because it leaves pointers
>> to freed up system memory pages in the GART tables of the drivers.
> So I wanted to review this series, and I can't reconcile your claim here
> with the demidlayering Dave has done. The driver patches here don't
> ouright undo what Dave has done, but that means the bug has been
> preexisting since forever (or is due to some other change?), and your
> commit message is a bit confusing here.
>
> The final patch just undoes the demidlayering from Dave, and I really
> don't see where there's even a functional change there.
>
> And even these patches here don't really change a hole lot with the
> calling sequence for at least final teardown: ttm_tt_destroy_common calls
> ttm_tt_unpopulate as the first thing, so at least there there's no change.
>
> Can you pls elaborate more clearly what exactly you're fixing and what
> exactly needs to be reordered and where this bug is from (commit sha1)? As
> is I'm playing detective and the evidence presented is extremely since and
> I can't reconcile it at all.
>
> I mean I know you don't like typing commit message and documentation, but
> it does get occasionally rather frustrating on the reviewer side if I have
> to interpolate between some very sparse hints for this stuff :-/

Yeah, when have seen the history it's rather obvious what's wrong here 
and I expected Dave to review it himself.

Previously we had three states in TTM for a tt object: Allocated -> 
Populated -> Bound which on destruction where done in the order unbind 
-> unpopulate -> free.

Dave moved handling of the bound state into the drivers since it is 
basically a driver decision and not a TTM decision what should be bound 
and what not (that part perfectly makes sense).

The problem is that he also moved doing the unbind into the free 
callback instead of the unpopulate callback. This result in stale page 
pointers in the GART if that unpopulate operation isn't immediately 
followed by a free.

Thinking more about it if we do populated->unpopulated->populated then 
we would also have stale pointers to the old pages which is even worse.

This is also not de-midlayering since we already have a proper 
ttm_tt_init()/ttm_tt_fini() functions which should work nicely for the 
tt object.

Christian.

> -Daniel
>
>> Signed-off-by: Christian König <christian.koenig@amd.com>
>> ---
>>   drivers/gpu/drm/vmwgfx/vmwgfx_ttm_buffer.c | 9 +++------
>>   1 file changed, 3 insertions(+), 6 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/vmwgfx/vmwgfx_ttm_buffer.c b/drivers/gpu/drm/vmwgfx/vmwgfx_ttm_buffer.c
>> index b0973c27e774..904031d03dbe 100644
>> --- a/drivers/gpu/drm/vmwgfx/vmwgfx_ttm_buffer.c
>> +++ b/drivers/gpu/drm/vmwgfx/vmwgfx_ttm_buffer.c
>> @@ -526,14 +526,9 @@ static void vmw_ttm_destroy(struct ttm_device *bdev, struct ttm_tt *ttm)
>>   	struct vmw_ttm_tt *vmw_be =
>>   		container_of(ttm, struct vmw_ttm_tt, dma_ttm);
>>   
>> -	vmw_ttm_unbind(bdev, ttm);
>>   	ttm_tt_destroy_common(bdev, ttm);
>>   	vmw_ttm_unmap_dma(vmw_be);
>> -	if (vmw_be->dev_priv->map_mode == vmw_dma_alloc_coherent)
>> -		ttm_tt_fini(&vmw_be->dma_ttm);
>> -	else
>> -		ttm_tt_fini(ttm);
>> -
>> +	ttm_tt_fini(ttm);
>>   	if (vmw_be->mob)
>>   		vmw_mob_destroy(vmw_be->mob);
>>   
>> @@ -578,6 +573,8 @@ static void vmw_ttm_unpopulate(struct ttm_device *bdev,
>>   						 dma_ttm);
>>   	unsigned int i;
>>   
>> +	vmw_ttm_unbind(bdev, ttm);
>> +
>>   	if (vmw_tt->mob) {
>>   		vmw_mob_destroy(vmw_tt->mob);
>>   		vmw_tt->mob = NULL;
>> -- 
>> 2.25.1
>>