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

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Message-Id: <20240904173645.1679-1-sj@kernel.org>
Date: Wed,  4 Sep 2024 10:36:45 -0700
From: SeongJae Park <sj@...nel.org>
To: "Liam R. Howlett" <Liam.Howlett@...cle.com>
Cc: SeongJae Park <sj@...nel.org>,
	Guenter Roeck <linux@...ck-us.net>,
	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

On Tue, 3 Sep 2024 22:43:40 -0400 "Liam R. Howlett" <Liam.Howlett@...cle.com> wrote:

> * Guenter Roeck <linux@...ck-us.net> [240903 21:54]:
> > On 9/3/24 18:18, SeongJae Park wrote:
> > > 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?
> > > 
> > > 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.
> 
> Again, I'd use the flags "MT_FLAGS_ALLOC_RANGE | MT_FLAGS_USE_RCU"
> because that gets you the gap tracking that may be necessary for tests
> in the future - it's closer to the MM_MT_FLAGS, so maybe some mm
> function you use depends on that.

Thank you for the nice suggestion with the rationales.  Just posted the v2
following it: https://lore.kernel.org/20240904172931.1284-1-sj@kernel.org

> 
> > > 
> > Same here. That is why I gave up after trying MT_FLAGS_ALLOC_RANGE and
> > "MT_FLAGS_ALLOC_RANGE | MT_FLAGS_USE_RCU". After all, I really don't know what
> > I am doing and was just playing around ... and there isn't really a good
> > explanation why initializing the maple tree with MT_FLAGS_ALLOC_RANGE (but not
> > MT_FLAGS_USE_RCU) would trigger rcu warnings.
> 
> Thanks, I'll add that to my list of things to do.

Thank you.  I agree that's somewhat we can visit separately.

FYI, I was also unable to reproduce rcu warnings with my v2 patch on my setup.
I will also try to use Guenter's more detailed repro
(https://lore.kernel.org/78880ac2-f7fe-4dc1-b2cb-25942fb0cacf@roeck-us.net).


Thanks,
SJ

> 
> Regards,
> Liam