Re: [PATCH 4/4] mm, notifier: Add a lockdep map for invalidate_range_start

[Jerome Glisse]

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>