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Message-ID: <b83651a0-5b24-4206-b860-cb54ffdf209b@roeck-us.net>
Date: Tue, 3 Sep 2024 21:27:22 -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 9/3/24 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
[ 6.240039] damon_test_three_regions_in_vmas+0x1ea/0x480
[ 6.240551] ? __pfx_kunit_generic_run_threadfn_adapter+0x10/0x10
[ 6.240712] kunit_try_run_case+0x93/0x190
[ 6.240850] ? __pfx_kunit_generic_run_threadfn_adapter+0x10/0x10
[ 6.240990] kunit_generic_run_threadfn_adapter+0x1c/0x40
[ 6.241124] kthread+0xdd/0x110
[ 6.241256] ? __pfx_kthread+0x10/0x10
[ 6.241368] ret_from_fork+0x2f/0x50
[ 6.241468] ? __pfx_kthread+0x10/0x10
[ 6.241573] ret_from_fork_asm+0x1a/0x30
[ 6.241765] </TASK>
[ 6.242180]
[ 6.242270] =============================
[ 6.242375] WARNING: suspicious RCU usage
[ 6.242478] 6.11.0-rc6-00029-gda66250b210f-dirty #1 Tainted: G N
[ 6.242634] -----------------------------
[ 6.242734] lib/maple_tree.c:788 suspicious rcu_dereference_check() usage!
[ 6.242955]
[ 6.242955] other info that might help us debug this:
[ 6.242955]
[ 6.243098]
[ 6.243098] rcu_scheduler_active = 2, debug_locks = 1
[ 6.243215] no locks held by kunit_try_catch/208.
[ 6.243331]
[ 6.243331] stack backtrace:
[ 6.243420] CPU: 0 UID: 0 PID: 208 Comm: kunit_try_catch Tainted: G N 6.11.0-rc6-00029-gda66250b210f-dirty #1
[ 6.243599] Tainted: [N]=TEST
[ 6.243665] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.16.3-0-ga6ed6b701f0a-prebuilt.qemu.org 04/01/2014
[ 6.243844] Call Trace:
[ 6.243907] <TASK>
[ 6.243961] dump_stack_lvl+0x9e/0xe0
[ 6.244032] lockdep_rcu_suspicious+0x145/0x1b0
[ 6.244121] mtree_range_walk+0x2b9/0x350
[ 6.244211] mas_walk+0x107/0x1d0
[ 6.244278] mas_find+0xb5/0x150
[ 6.244341] __damon_va_three_regions+0x7e/0x130
[ 6.244445] damon_test_three_regions_in_vmas+0x1ea/0x480
[ 6.244770] ? __pfx_kunit_generic_run_threadfn_adapter+0x10/0x10
[ 6.244865] kunit_try_run_case+0x93/0x190
[ 6.244952] ? __pfx_kunit_generic_run_threadfn_adapter+0x10/0x10
[ 6.245044] kunit_generic_run_threadfn_adapter+0x1c/0x40
[ 6.245130] kthread+0xdd/0x110
[ 6.245191] ? __pfx_kthread+0x10/0x10
[ 6.245262] ret_from_fork+0x2f/0x50
[ 6.245326] ? __pfx_kthread+0x10/0x10
[ 6.245394] ret_from_fork_asm+0x1a/0x30
[ 6.245497] </TASK>
[ 6.246605] # damon_test_three_regions_in_vmas: pass:1 fail:0 skip:0 total:1
[ 6.246668] ok 1 damon_test_three_regions_in_vmas
This is with
diff --git a/mm/damon/vaddr-test.h b/mm/damon/vaddr-test.h
index 83626483f82b..a339d117150f 100644
--- a/mm/damon/vaddr-test.h
+++ b/mm/damon/vaddr-test.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");
I'll put it all together and make it available, but that will have to wait until
tomorrow.
Thanks,
Guenter
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