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

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Message-ID: <34f3e9bd-9323-43cc-8371-00d8847d8664@roeck-us.net>
Date: Wed, 4 Sep 2024 11:41:28 -0700
From: Guenter Roeck <linux@...ck-us.net>
To: SeongJae Park <sj@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>
Cc: "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 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 ?

Thanks,
Guenter

> - Fix a typo: s/celar/clear/ (Guenter)
> 
>   mm/damon/tests/vaddr-kunit.h | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/damon/tests/vaddr-kunit.h b/mm/damon/tests/vaddr-kunit.h
> index 83626483f82b..a339d117150f 100644
> --- 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_flags(&mm.mm_mt, MT_FLAGS_ALLOC_RANGE | MT_FLAGS_USE_RCU);
>   	if (__link_vmas(&mm.mm_mt, vmas, ARRAY_SIZE(vmas)))
>   		kunit_skip(test, "Failed to create VMA tree");
>