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

[Guenter Roeck <linux@roeck-us.net>]

On 9/3/24 18:18, SeongJae Park wrote:
> On Tue,  3 Sep 2024 17:58:15 -0700 SeongJae Park <sj@kernel.org> wrote:
> 
>> On Tue, 3 Sep 2024 20:48:53 -0400 "Liam R. Howlett" <Liam.Howlett@oracle.com> wrote:
>>
>>> * SeongJae Park <sj@kernel.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.
> 
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.

Guenter