linux-kernel - Re: [PATCH v2] mm/damon/tests/vaddr-kunit: init maple tree without MT_FLAGS_LOCK

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Message-Id: <20240904185347.2253-1-sj@kernel.org>
Date: Wed,  4 Sep 2024 11:53:47 -0700
From: SeongJae Park <sj@...nel.org>
To: Guenter Roeck <linux@...ck-us.net>
Cc: SeongJae Park <sj@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Liam R. Howlett" <Liam.Howlett@...cle.com>,
	David Hildenbrand <david@...hat.com>,
	"Matthew Wilcox (Oracle)" <willy@...radead.org>,
	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 v2] mm/damon/tests/vaddr-kunit: init maple tree without MT_FLAGS_LOCK_EXTERN

On Wed, 4 Sep 2024 11:41:28 -0700 Guenter Roeck <linux@...ck-us.net> wrote:

> On 9/4/24 10:29, SeongJae Park wrote:
> > 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 clear when spinlock debugging is turned on, since it
> > reports spinlock bad magic bug.
> > 
> > Fix the issue by excluding MT_FLAGS_LOCK_EXTERN from the maple tree
> > initialization flags.  Note that we don't use empty flags to make it
> > further similar to the usage of mm maple tree, and to be prepared for
> > possible future changes, as suggested by Liam.
> > 
> > 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")
> > Suggested-by: Liam R. Howlett <Liam.Howlett@...cle.com>
> > Signed-off-by: SeongJae Park <sj@...nel.org>
> > ---
> > Changes from v1
> > (https://lore.kernel.org/20240904004534.1189-1-sj@kernel.org)
> > - Keep lock usage and update the initialization flags (Liam)
> 
> Not sure I understand how this is better. Is the resulting rcu warning
> considered to be irrelevant or a separate problem ?

I believe it's a separate potential problem (warning) that deserves a look.
Meanwhile, this patch is fixing an issue that is definitely a bug, so better to
fix right now, imho.


Thanks,
SJ

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