Re: [PATCH 4/5] mm, notifier: Add a lockdep map for invalidate_range_start

[Jason Gunthorpe]

On Wed, Aug 14, 2019 at 10:20:26PM +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 was thinking about doing something like this..

IMHO only range_end needs annotation, the other ops are either already
non-sleeping or only used by KVM.

BTW, I have found it strange that i915 only uses
invalidate_range_start. Not really sure how it is able to do
that. Would love to know the answer :)

> Reviewed-by: JÃrÃme Glisse <jglisse@xxxxxxxxxx>
> Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxxx>
>  include/linux/mmu_notifier.h | 6 ++++++
>  mm/mmu_notifier.c            | 7 +++++++
>  2 files changed, 13 insertions(+)
> 
> diff --git a/include/linux/mmu_notifier.h b/include/linux/mmu_notifier.h
> index b6c004bd9f6a..9dd38c32fc53 100644
> +++ b/include/linux/mmu_notifier.h
> @@ -42,6 +42,10 @@ enum mmu_notifier_event {
>  
>  #ifdef CONFIG_MMU_NOTIFIER
>  
> +#ifdef CONFIG_LOCKDEP
> +extern struct lockdep_map __mmu_notifier_invalidate_range_start_map;
> +#endif

I wonder what the trade off is having a global map vs a map in each
mmu_notifier_mm ?

>  /*
>   * The mmu notifier_mm structure is allocated and installed in
>   * mm->mmu_notifier_mm inside the mm_take_all_locks() protected
> @@ -310,10 +314,12 @@ static inline void mmu_notifier_change_pte(struct mm_struct *mm,
>  static inline void
>  mmu_notifier_invalidate_range_start(struct mmu_notifier_range *range)
>  {
> +	lock_map_acquire(&__mmu_notifier_invalidate_range_start_map);
>  	if (mm_has_notifiers(range->mm)) {
>  		range->flags |= MMU_NOTIFIER_RANGE_BLOCKABLE;
>  		__mmu_notifier_invalidate_range_start(range);
>  	}
> +	lock_map_release(&__mmu_notifier_invalidate_range_start_map);
>  }

Also range_end should have this too - it has all the same
constraints. I think it can share the map. So 'range_start_map' is
probably not the right name.

It may also make some sense to do a dummy acquire/release under the
mm_take_all_locks() to forcibly increase map coverage and reduce the
scenario complexity required to hit bugs.

And if we do decide on the reclaim thing in my other email then the
reclaim dependency can be reliably injected by doing:

 fs_reclaim_acquire();
 lock_map_acquire(&__mmu_notifier_invalidate_range_start_map);
 lock_map_release(&__mmu_notifier_invalidate_range_start_map);
 fs_reclaim_release();

If I understand lockdep properly..

Jason