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

[Daniel Vetter <daniel.vetter@ffwll.ch>]

On Tue, May 21, 2019 at 5:41 PM Jerome Glisse <jglisse@redhat.com> 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

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.

> 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.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch