[Linaro-mm-sig] [PATCH 1/5] dma-buf: add optional invalidate_mappings callback v2

[Daniel Vetter]

On Tue, Mar 27, 2018 at 09:35:17AM +0200, Christian König wrote:
> Am 26.03.2018 um 17:42 schrieb Jerome Glisse:
> > On Mon, Mar 26, 2018 at 10:01:21AM +0200, Daniel Vetter wrote:
> > > On Thu, Mar 22, 2018 at 10:58:55AM +0100, Christian König wrote:
> > > > Am 22.03.2018 um 08:18 schrieb Daniel Vetter:
> > > > [SNIP]
> > > > Key take away from that was that you can't take any locks from neither the
> > > > MMU notifier nor the shrinker you also take while calling kmalloc (or
> > > > simpler speaking get_user_pages()).
> > > > 
> > > > Additional to that in the MMU or shrinker callback all different kinds of
> > > > locks might be held, so you basically can't assume that you do thinks like
> > > > recursive page table walks or call dma_unmap_anything.
> > > That sounds like a design bug in mmu_notifiers, since it would render them
> > > useless for KVM. And they were developed for that originally. I think I'll
> > > chat with Jerome to understand this, since it's all rather confusing.
> > Doing dma_unmap() during mmu_notifier callback should be fine, it was last
> > time i check. However there is no formal contract that it is ok to do so.
> 
> As I said before dma_unmap() isn't the real problem here.
> 
> The issues is more that you can't take a lock in the MMU notifier which you
> would also take while allocating memory without GFP_NOIO.
> 
> That makes it rather tricky to do any command submission, e.g. you need to
> grab all the pages/memory/resources prehand, then make sure that you don't
> have a MMU notifier running concurrently and do the submission.
> 
> If any of the prerequisites isn't fulfilled we need to restart the
> operation.

Yeah we're hitting all that epic amount of fun now, after a chat with
Jerome yesterady. I guess we'll figure out what we're coming up with.

> > [SNIP]
> > A slightly better solution is using atomic counter:
> >    driver_range_start() {
> >      atomic_inc(&mydev->notifier_count);
> ...
> 
> Yeah, that is exactly what amdgpu is doing now. Sorry if my description
> didn't made that clear.
> 
> > I would like to see driver using same code, as it means one place to fix
> > issues. I had for a long time on my TODO list doing the above conversion
> > to amd or radeon kernel driver. I am pushing up my todo list hopefully in
> > next few weeks i can send an rfc so people can have a real sense of how
> > it can look.
> 
> Certainly a good idea, but I think we might have that separate to HMM.
> 
> TTM suffered really from feature overload, e.g. trying to do everything in a
> single subsystem. And it would be rather nice to have coherent userptr
> handling for GPUs as separate feature.

TTM suffered from being a midlayer imo, not from doing too much. HMM is
apparently structured like a toolbox (despite its documentation claiming
otherwise), so you can pick&choose freely.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch