Message-ID: <gaouhgel2kmzkm7pofpkjkt5ya3a44rwsal74zsgzh76xzh4qo@ne6sqvzj5qor>
Date: Wed, 4 Sep 2024 15:26:11 -0400
From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
To: Guenter Roeck <linux@...ck-us.net>
Cc: 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

* 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;

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