Re: [PATCH] drm/ttm: stop warning on TT shrinker failure

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

Am 23.03.21 um 13:37 schrieb Michal Hocko:
> On Tue 23-03-21 13:21:32, Christian König wrote:
>> Am 23.03.21 um 13:04 schrieb Michal Hocko:
>>> On Tue 23-03-21 12:48:58, Christian König wrote:
>>>> Am 23.03.21 um 12:28 schrieb Daniel Vetter:
>>>>> On Tue, Mar 23, 2021 at 08:38:33AM +0100, Michal Hocko wrote:
>>>>>> On Mon 22-03-21 20:34:25, Christian König wrote:
>>> [...]
>>>>>>> My only concern is that if I could rely on memalloc_no* being used we could
>>>>>>> optimize this quite a bit further.
>>>>>> Yes you can use the scope API and you will be guaranteed that _any_
>>>>>> allocation from the enclosed context will inherit GFP_NO* semantic.
>>>> The question is if this is also guaranteed the other way around?
>>>>
>>>> In other words if somebody calls get_free_page(GFP_NOFS) are the context
>>>> flags set as well?
>>> gfp mask is always restricted in the page allocator. So say you have
>>> noio scope context and call get_free_page/kmalloc(GFP_NOFS) then the
>>> scope would restrict the allocation flags to GFP_NOIO (aka drop
>>> __GFP_IO). For further details, have a look at current_gfp_context
>>> and its callers.
>>>
>>> Does this answer your question?
>> But what happens if you don't have noio scope and somebody calls
>> get_free_page(GFP_NOFS)?
> Then this will be a regular NOFS request. Let me repeat scope API will
> further restrict any requested allocation mode.

Ok, got it.

>
>> Is then the noio scope added automatically? And is it possible that the
>> shrinker gets called without noio scope even we would need it?
> Here you have lost me again.
>
>>>>>> I think this is where I don't get yet what Christian tries to do: We
>>>>>> really shouldn't do different tricks and calling contexts between direct
>>>>>> reclaim and kswapd reclaim. Otherwise very hard to track down bugs are
>>>>>> pretty much guaranteed. So whether we use explicit gfp flags or the
>>>>>> context apis, result is exactly the same.
>>>> Ok let us recap what TTMs TT shrinker does here:
>>>>
>>>> 1. We got memory which is not swapable because it might be accessed by the
>>>> GPU at any time.
>>>> 2. Make sure the memory is not accessed by the GPU and driver need to grab a
>>>> lock before they can make it accessible again.
>>>> 3. Allocate a shmem file and copy over the not swapable pages.
>>> This is quite tricky because the shrinker operates in the PF_MEMALLOC
>>> context so such an allocation would be allowed to completely deplete
>>> memory unless you explicitly mark that context as __GFP_NOMEMALLOC.
>> Thanks, exactly that was one thing I was absolutely not sure about. And yes
>> I agree that this is really tricky.
>>
>> Ideally I would like to be able to trigger swapping out the shmem page I
>> allocated immediately after doing the copy.
> So let me try to rephrase to make sure I understand. You would like to
> swap out the existing content from the shrinker and you use shmem as a
> way to achieve that. The swapout should happen at the time of copying
> (shrinker context) or shortly afterwards?
>
> So effectively to call pageout() on the shmem page after the copy?

Yes, exactly that.

>> This way I would only need a single page for the whole shrink operation at
>> any given time.
> What do you mean by that? You want the share the same shmem page for
> other copy+swapout?

Correct, yes.

The idea is that we can swap out the content of a full GPU buffer object 
this way to give the backing store of the object back to the core memory 
managment.

Regards,
Christian.