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

[Michal Hocko <mhocko@suse.com>]

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?

> > > 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. Also
note that if the allocation cannot succeed it will not trigger reclaim
again because you are already called from the reclaim context.

> 4. Free the not swapable/reclaimable pages.
> 
> The pages we got from the shmem file are easily swapable to disk after the
> copy is completed. But only if IO is not already blocked because the
> shrinker was called from an allocation restricted by GFP_NOFS or GFP_NOIO.

Sorry for being dense here but I still do not follow the actual problem
(well, except for the above mentioned one). Is the sole point of this to
emulate a GFP_NO* allocation context and see how shrinker behaves? 
-- 
Michal Hocko
SUSE Labs