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

[SeongJae Park]

On Tue,  3 Sep 2024 17:58:15 -0700 SeongJae Park <sj@xxxxxxxxxx> wrote:

> On Tue, 3 Sep 2024 20:48:53 -0400 "Liam R. Howlett" <Liam.Howlett@xxxxxxxxxx> wrote:
> 
> > * SeongJae Park <sj@xxxxxxxxxx> [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?

I was actually thinking replacing the mt_init_flags() with mt_init(), which
same to mt_init_flags() with zero flag, like below.

```
--- a/mm/damon/tests/vaddr-kunit.h
+++ b/mm/damon/tests/vaddr-kunit.h
@@ -77,7 +77,7 @@ static void damon_test_three_regions_in_vmas(struct kunit *test)
                (struct vm_area_struct) {.vm_start = 307, .vm_end = 330},
        };

-       mt_init_flags(&mm.mm_mt, MM_MT_FLAGS);
+       mt_init(&mm.mm_mt);
        if (__link_vmas(&mm.mm_mt, vmas, ARRAY_SIZE(vmas)))
                kunit_skip(test, "Failed to create VMA tree");
```

And just confirmed it also convinces the reproducer.  But because I'm obviously
not familiar with maple tree, would like to hear some comments from Liam or
others first.

FYI, I ended up writing v1 to simply remove lock usage based on my humble
understanding of the documetnation.

    The maple tree uses a spinlock by default, but external locks can be used for
    tree updates as well.  To use an external lock, the tree must be initialized
    with the ``MT_FLAGS_LOCK_EXTERN flag``, this is usually done with the
    MTREE_INIT_EXT() #define, which takes an external lock as an argument.

(from Documentation/core-api/maple_tree.rst)

I was thinking the fact that the test code is executed in single thread is same
to use of external lock.  I will be happy to learn if I missed something.


Thanks,
SJ

> 
> 
> Thanks,
> SJ
> 
> > 
> > > 
> > > Reported-by: Guenter Roeck <linux@xxxxxxxxxxxx>
> > > Closes: https://lore.kernel.org/1453b2b2-6119-4082-ad9e-f3c5239bf87e@xxxxxxxxxxxx
> > > Fixes: d0cf3dd47f0d ("damon: convert __damon_va_three_regions to use the VMA iterator")
> > > Signed-off-by: SeongJae Park <sj@xxxxxxxxxx>
> > > ---
> > >  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
> > >