On Tue, May 21, 2019 at 06:00:36PM +0200, Daniel Vetter wrote: > On Tue, May 21, 2019 at 5:41 PM Jerome Glisse <jglisse@xxxxxxxxxx> wrote: > > > > On Mon, May 20, 2019 at 11:39:45PM +0200, Daniel Vetter wrote: > > > This is a similar idea to the fs_reclaim fake lockdep lock. It's > > > fairly easy to provoke a specific notifier to be run on a specific > > > range: Just prep it, and then munmap() it. > > > > > > A bit harder, but still doable, is to provoke the mmu notifiers for > > > all the various callchains that might lead to them. But both at the > > > same time is really hard to reliable hit, especially when you want to > > > exercise paths like direct reclaim or compaction, where it's not > > > easy to control what exactly will be unmapped. > > > > > > By introducing a lockdep map to tie them all together we allow lockdep > > > to see a lot more dependencies, without having to actually hit them > > > in a single challchain while testing. > > > > > > Aside: Since I typed this to test i915 mmu notifiers I've only rolled > > > this out for the invaliate_range_start callback. If there's > > > interest, we should probably roll this out to all of them. But my > > > undestanding of core mm is seriously lacking, and I'm not clear on > > > whether we need a lockdep map for each callback, or whether some can > > > be shared. > > > > I need to read more on lockdep but it is legal to have mmu notifier > > invalidation within each other. For instance when you munmap you > > might split a huge pmd and it will trigger a second invalidate range > > while the munmap one is not done yet. Would that trigger the lockdep > > here ? > > Depends how it's nesting. I'm wrapping the annotation only just around > the individual mmu notifier callback, so if the nesting is just > - munmap starts > - invalidate_range_start #1 > - we noticed that there's a huge pmd we need to split > - invalidate_range_start #2 > - invalidate_reange_end #2 > - invalidate_range_end #1 > - munmap is done Yeah this is how it looks. All the callback from range_start #1 would happens before range_start #2 happens so we should be fine. > > But if otoh it's ok to trigger the 2nd invalidate range from within an > mmu_notifier->invalidate_range_start callback, then lockdep will be > pissed about that. No that would be illegal for a callback to do that. There is no existing callback that would do that at least AFAIK. So we can just say that it is illegal. I would not see the point. > > > Worst case i can think of is 2 invalidate_range_start chain one after > > the other. I don't think you can triggers a 3 levels nesting but maybe. > > Lockdep has special nesting annotations. I think it'd be more an issue > of getting those funneled through the entire call chain, assuming we > really need that. I think we are fine. So this patch looks good. Reviewed-by: Jérôme Glisse <jglisse@xxxxxxxxxx>
