linux-kernel - Re: [PATCH] mm/damon/tests/vaddr-kunit: don't use mas_lock for MM_MT

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Message-ID: <whdjeq6qpccj6ms4wgiyjcnizht4nl5qbt7rbaeqfwzt67smxt@vvduwpqcuizl>
Date: Tue, 3 Sep 2024 22:31:21 -0400
From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
To: SeongJae Park <sj@...nel.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        David Hildenbrand <david@...hat.com>,
        Brendan Higgins <brendanhiggins@...gle.com>,
        David Gow <davidgow@...gle.com>, damon@...ts.linux.dev,
        linux-mm@...ck.org, kunit-dev@...glegroups.com,
        linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org,
        Guenter Roeck <linux@...ck-us.net>
Subject: Re: [PATCH] mm/damon/tests/vaddr-kunit: don't use mas_lock for
 MM_MT_FLAGS-initialized maple tree

* SeongJae Park <sj@...nel.org> [240903 21:18]:
> On Tue,  3 Sep 2024 17:58:15 -0700 SeongJae Park <sj@...nel.org> wrote:
> 
> > On Tue, 3 Sep 2024 20:48:53 -0400 "Liam R. Howlett" <Liam.Howlett@...cle.com> wrote:
> > 
> > > * SeongJae Park <sj@...nel.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?

That would be the lockdep complaining, so that's good.

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

Yes.  This will use the spinlock which should fix your issue, but it
will use a different style of maple tree.

Perhaps use MT_FLAGS_ALLOC_RANGE to use the same type of maple tree, if
you ever add threading you will want the rcu flag as well
(MT_FLAGS_USE_RCU).

I would recommend those two and just use the spinlock.

> 
> ```
> --- 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@...ck-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@...nel.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
> > > >