On 9/3/24 19:31, Liam R. Howlett wrote:
* SeongJae Park <sj@xxxxxxxxxx> [240903 21:18]:
On Tue, 3 Sep 2024 17:58:15 -0700 SeongJae Park <sj@xxxxxxxxxx> wrote:
On Tue, 3 Sep 2024 20:48:53 -0400 "Liam R. Howlett" <Liam.Howlett@xxxxxxxxxx> wrote:
* SeongJae Park <sj@xxxxxxxxxx> [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.
Guenter
```
--- 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.
FYI, I ended up writing v1 to simply remove lock usage based on my humble
understanding of the documetnation.
The maple tree uses a spinlock by default, but external locks can be used for
tree updates as well. To use an external lock, the tree must be initialized
with the ``MT_FLAGS_LOCK_EXTERN flag``, this is usually done with the
MTREE_INIT_EXT() #define, which takes an external lock as an argument.
(from Documentation/core-api/maple_tree.rst)
I was thinking the fact that the test code is executed in single thread is same
to use of external lock. I will be happy to learn if I missed something.
Thanks,
SJ
Thanks,
SJ
Reported-by: Guenter Roeck <linux@xxxxxxxxxxxx>
Closes: https://lore.kernel.org/1453b2b2-6119-4082-ad9e-f3c5239bf87e@xxxxxxxxxxxx
Fixes: d0cf3dd47f0d ("damon: convert __damon_va_three_regions to use the VMA iterator")
Signed-off-by: SeongJae Park <sj@xxxxxxxxxx>
---
mm/damon/tests/vaddr-kunit.h | 10 +++-------
1 file changed, 3 insertions(+), 7 deletions(-)
diff --git a/mm/damon/tests/vaddr-kunit.h b/mm/damon/tests/vaddr-kunit.h
index 83626483f82b..c6c7e0e0ab07 100644
--- a/mm/damon/tests/vaddr-kunit.h
+++ b/mm/damon/tests/vaddr-kunit.h
@@ -17,23 +17,19 @@
static int __link_vmas(struct maple_tree *mt, struct vm_area_struct *vmas,
ssize_t nr_vmas)
{
- int i, ret = -ENOMEM;
+ int i;
MA_STATE(mas, mt, 0, 0);
if (!nr_vmas)
return 0;
- mas_lock(&mas);
for (i = 0; i < nr_vmas; i++) {
mas_set_range(&mas, vmas[i].vm_start, vmas[i].vm_end - 1);
if (mas_store_gfp(&mas, &vmas[i], GFP_KERNEL))
- goto failed;
+ return -ENOMEM;
}
- ret = 0;
-failed:
- mas_unlock(&mas);
- return ret;
+ return 0;
}
/*
--
2.39.2
