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Message-ID: <274f253a-e44e-431b-9dd3-a499843be96f@roeck-us.net>
Date: Wed, 4 Sep 2024 12:56:33 -0700
From: Guenter Roeck <linux@...ck-us.net>
To: "Liam R. Howlett" <Liam.Howlett@...cle.com>, SeongJae Park
<sj@...nel.org>, 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 9/4/24 12:26, Liam R. Howlett wrote:
> * Guenter Roeck <linux@...ck-us.net> [240904 00:27]:
>> On 9/3/24 20:36, Liam R. Howlett wrote:
>>> * Guenter Roeck <linux@...ck-us.net> [240903 22:38]:
>>>> On 9/3/24 19:31, Liam R. Howlett wrote:
>>>>> * SeongJae Park <sj@...nel.org> [240903 21:18]:
>>>>>> 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?
>>>>>
>>>>> 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.
>>>>
>>>
>>> I am running ./tools/testing/kunit/kunit.py run '*damon*' --arch x86_64 --raw
>>> with:
>>> CONFIG_LOCKDEP=y
>>> CONFIG_DEBUG_SPINLOCK=y
>>>
>>> and I don't have any issue with locking in the existing code. How do I
>>> recreate this issue?
>>>
>>
>> I tested again, and I still see
>>
>>
>> [ 6.233483] ok 4 damon
>> [ 6.234190] KTAP version 1
>> [ 6.234263] # Subtest: damon-operations
>> [ 6.234335] # module: vaddr
>> [ 6.234384] 1..6
>> [ 6.235726]
>> [ 6.235931] =============================
>> [ 6.236018] WARNING: suspicious RCU usage
>> [ 6.236280] 6.11.0-rc6-00029-gda66250b210f-dirty #1 Tainted: G N
>> [ 6.236398] -----------------------------
>> [ 6.236474] lib/maple_tree.c:832 suspicious rcu_dereference_check() usage!
>> [ 6.236579]
>> [ 6.236579] other info that might help us debug this:
>> [ 6.236579]
>> [ 6.236738]
>> [ 6.236738] rcu_scheduler_active = 2, debug_locks = 1
>> [ 6.237039] no locks held by kunit_try_catch/208.
>> [ 6.237166]
>> [ 6.237166] stack backtrace:
>> [ 6.237385] CPU: 0 UID: 0 PID: 208 Comm: kunit_try_catch Tainted: G N 6.11.0-rc6-00029-gda66250b210f-dirty #1
>> [ 6.237629] Tainted: [N]=TEST
>> [ 6.237714] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.16.3-0-ga6ed6b701f0a-prebuilt.qemu.org 04/01/2014
>> [ 6.238065] Call Trace:
>> [ 6.238233] <TASK>
>> [ 6.238547] dump_stack_lvl+0x9e/0xe0
>> [ 6.239473] lockdep_rcu_suspicious+0x145/0x1b0
>> [ 6.239621] mas_walk+0x19f/0x1d0
>> [ 6.239765] mas_find+0xb5/0x150
>> [ 6.239873] __damon_va_three_regions+0x7e/0x130
>
> This function isn't taking the rcu read lock while iterating the tree.
>
> Try this:
>
> diff --git a/mm/damon/vaddr.c b/mm/damon/vaddr.c
> index b0e8b361891d..08cfd22b5249 100644
> --- a/mm/damon/vaddr.c
> +++ b/mm/damon/vaddr.c
> @@ -126,6 +126,7 @@ static int __damon_va_three_regions(struct mm_struct *mm,
> * If this is too slow, it can be optimised to examine the maple
> * tree gaps.
> */
> + rcu_read_lock();
> for_each_vma(vmi, vma) {
> unsigned long gap;
>
> @@ -146,6 +147,7 @@ static int __damon_va_three_regions(struct mm_struct *mm,
> next:
> prev = vma;
> }
> + rcu_read_unlock();
>
> if (!sz_range(&second_gap) || !sz_range(&first_gap))
> return -EINVAL;
>
Yes, that fixes the problem for me.
Thanks,
Guenter
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