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

From: Liam R. Howlett
Date: Tue Sep 03 2024 - 22:47:04 EST


* Guenter Roeck <linux@xxxxxxxxxxxx> [240903 22:38]:
> On 9/3/24 19:31, Liam R. Howlett wrote:
> > * SeongJae Park <sj@xxxxxxxxxx> [240903 21:18]:
> > > 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?
> >
> > 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.

Just to be clear, you changed the init flags and kept the mas_lock() and
mas_unlock() ?

Thanks,
Liam