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

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Message-ID: <54zu64vxrhdxr4wtmwbewga44shu4f7lz4ffx2hxhag46b56hn@qgrgfrb6mhbv>
Date: Tue, 3 Sep 2024 23:36:41 -0400
From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
To: Guenter Roeck <linux@...ck-us.net>
Cc: SeongJae Park <sj@...nel.org>, 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
Subject: Re: [PATCH] mm/damon/tests/vaddr-kunit: don't use mas_lock for
 MM_MT_FLAGS-initialized maple tree

* Guenter Roeck <linux@...ck-us.net> [240903 22:38]:
> On 9/3/24 19:31, Liam R. Howlett wrote:
> > * 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.
> > 
> 
> I tried that (MT_FLAGS_ALLOC_RANGE | MT_FLAGS_USE_RCU). it also triggers
> the suspicious RCU usage message.
> 

I am running ./tools/testing/kunit/kunit.py run '*damon*' --arch x86_64 --raw 
with:
CONFIG_LOCKDEP=y
CONFIG_DEBUG_SPINLOCK=y

and I don't have any issue with locking in the existing code.  How do I
recreate this issue?

Thanks,
Liam