Re: [PATCH] mm/damon/tests/vaddr-kunit: don't use mas_lock for MM_MT_FLAGS-initialized maple tree

[SeongJae Park <sj@kernel.org>]

On Tue, 3 Sep 2024 20:48:53 -0400 "Liam R. Howlett" <Liam.Howlett@oracle.com> wrote:

> * SeongJae Park <sj@kernel.org> [240903 20:45]:
> > damon_test_three_regions_in_vmas() initializes a maple tree with
> > MM_MT_FLAGS.  The flags contains MT_FLAGS_LOCK_EXTERN, which means
> > mt_lock of the maple tree will not be used.  And therefore the maple
> > tree initialization code skips initialization of the mt_lock.  However,
> > __link_vmas(), which adds vmas for test to the maple tree, uses the
> > mt_lock.  In other words, the uninitialized spinlock is used.  The
> > problem becomes celar when spinlock debugging is turned on, since it
> > reports spinlock bad magic bug.  Fix the issue by not using the mt_lock
> > as promised.
> 
> You can't do this, lockdep will tell you this is wrong.

Hmm, but lockdep was silence on my setup?

> We need a lock and to use the lock for writes.

This code is executed by a single-thread test code.  Do we still need the lock?

> 
> I'd suggest using different flags so the spinlock is used.

The reporter mentioned simply dropping MT_FLAGS_LOCK_EXTERN from the flags
causes suspicious RCU usage message.  May I ask if you have a suggestion of
better flags?


Thanks,
SJ

> 
> > 
> > Reported-by: Guenter Roeck <linux@roeck-us.net>
> > Closes: https://lore.kernel.org/1453b2b2-6119-4082-ad9e-f3c5239bf87e@roeck-us.net
> > Fixes: d0cf3dd47f0d ("damon: convert __damon_va_three_regions to use the VMA iterator")
> > Signed-off-by: SeongJae Park <sj@kernel.org>
> > ---
> >  mm/damon/tests/vaddr-kunit.h | 10 +++-------
> >  1 file changed, 3 insertions(+), 7 deletions(-)
> > 
> > diff --git a/mm/damon/tests/vaddr-kunit.h b/mm/damon/tests/vaddr-kunit.h
> > index 83626483f82b..c6c7e0e0ab07 100644
> > --- a/mm/damon/tests/vaddr-kunit.h
> > +++ b/mm/damon/tests/vaddr-kunit.h
> > @@ -17,23 +17,19 @@
> >  static int __link_vmas(struct maple_tree *mt, struct vm_area_struct *vmas,
> >  			ssize_t nr_vmas)
> >  {
> > -	int i, ret = -ENOMEM;
> > +	int i;
> >  	MA_STATE(mas, mt, 0, 0);
> >  
> >  	if (!nr_vmas)
> >  		return 0;
> >  
> > -	mas_lock(&mas);
> >  	for (i = 0; i < nr_vmas; i++) {
> >  		mas_set_range(&mas, vmas[i].vm_start, vmas[i].vm_end - 1);
> >  		if (mas_store_gfp(&mas, &vmas[i], GFP_KERNEL))
> > -			goto failed;
> > +			return -ENOMEM;
> >  	}
> >  
> > -	ret = 0;
> > -failed:
> > -	mas_unlock(&mas);
> > -	return ret;
> > +	return 0;
> >  }
> >  
> >  /*
> > -- 
> > 2.39.2
> >