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

[Jerome Glisse]

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 ?

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.

Cheers,
Jérôme