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

[Michal Hocko <mhocko@suse.com>]

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.

> 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?
 
> 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?
-- 
Michal Hocko
SUSE Labs