[v2] mm/damon/tests/vaddr-kunit: init maple tree without MT_FLAGS_LOCK_EXTERN

Commit Message

SeongJae Park Sept. 4, 2024, 5:29 p.m. UTC

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 clear when spinlock debugging is turned on, since it
reports spinlock bad magic bug.

Fix the issue by excluding MT_FLAGS_LOCK_EXTERN from the maple tree
initialization flags.  Note that we don't use empty flags to make it
further similar to the usage of mm maple tree, and to be prepared for
possible future changes, as suggested by Liam.

Reported-by: Guenter Roeck <[e-mail]>
Closes: https://lore.kernel.org/[e-mail]
Fixes: d0cf3dd47f0d ("damon: convert __damon_va_three_regions to use the VMA iterator")
Suggested-by: Liam R. Howlett <[e-mail]>
Signed-off-by: SeongJae Park <[e-mail]>
---
Changes from v1
(https://lore.kernel.org/[e-mail])
- Keep lock usage and update the initialization flags (Liam)
- Fix a typo: s/celar/clear/ (Guenter)

 mm/damon/tests/vaddr-kunit.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Comments

Guenter Roeck Sept. 4, 2024, 6:41 p.m. UTC | #1

On 9/4/24 10:29, SeongJae Park wrote:
> 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 clear when spinlock debugging is turned on, since it
> reports spinlock bad magic bug.
> 
> Fix the issue by excluding MT_FLAGS_LOCK_EXTERN from the maple tree
> initialization flags.  Note that we don't use empty flags to make it
> further similar to the usage of mm maple tree, and to be prepared for
> possible future changes, as suggested by Liam.
> 
> Reported-by: Guenter Roeck <[e-mail]>
> Closes: https://lore.kernel.org/[e-mail]
> Fixes: d0cf3dd47f0d ("damon: convert __damon_va_three_regions to use the VMA iterator")
> Suggested-by: Liam R. Howlett <[e-mail]>
> Signed-off-by: SeongJae Park <[e-mail]>
> ---
> Changes from v1
> (https://lore.kernel.org/[e-mail])
> - Keep lock usage and update the initialization flags (Liam)

Not sure I understand how this is better. Is the resulting rcu warning
considered to be irrelevant or a separate problem ?

Thanks,
Guenter

> - Fix a typo: s/celar/clear/ (Guenter)
> 
>   mm/damon/tests/vaddr-kunit.h | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/damon/tests/vaddr-kunit.h b/mm/damon/tests/vaddr-kunit.h
> index 83626483f82b..a339d117150f 100644
> --- 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_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");
>

SeongJae Park Sept. 4, 2024, 6:53 p.m. UTC | #2

On Wed, 4 Sep 2024 11:41:28 -0700 Guenter Roeck <[e-mail]> wrote:

> On 9/4/24 10:29, SeongJae Park wrote:
> > 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 clear when spinlock debugging is turned on, since it
> > reports spinlock bad magic bug.
> > 
> > Fix the issue by excluding MT_FLAGS_LOCK_EXTERN from the maple tree
> > initialization flags.  Note that we don't use empty flags to make it
> > further similar to the usage of mm maple tree, and to be prepared for
> > possible future changes, as suggested by Liam.
> > 
> > Reported-by: Guenter Roeck <[e-mail]>
> > Closes: https://lore.kernel.org/[e-mail]
> > Fixes: d0cf3dd47f0d ("damon: convert __damon_va_three_regions to use the VMA iterator")
> > Suggested-by: Liam R. Howlett <[e-mail]>
> > Signed-off-by: SeongJae Park <[e-mail]>
> > ---
> > Changes from v1
> > (https://lore.kernel.org/[e-mail])
> > - Keep lock usage and update the initialization flags (Liam)
> 
> Not sure I understand how this is better. Is the resulting rcu warning
> considered to be irrelevant or a separate problem ?

I believe it's a separate potential problem (warning) that deserves a look.
Meanwhile, this patch is fixing an issue that is definitely a bug, so better to
fix right now, imho.


Thanks,
SJ

[...]

Liam R. Howlett Sept. 4, 2024, 7:28 p.m. UTC | #3

* SeongJae Park <[e-mail]> [240904 14:53]:
> On Wed, 4 Sep 2024 11:41:28 -0700 Guenter Roeck <[e-mail]> wrote:
> 
> > On 9/4/24 10:29, SeongJae Park wrote:
> > > 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 clear when spinlock debugging is turned on, since it
> > > reports spinlock bad magic bug.
> > > 
> > > Fix the issue by excluding MT_FLAGS_LOCK_EXTERN from the maple tree
> > > initialization flags.  Note that we don't use empty flags to make it
> > > further similar to the usage of mm maple tree, and to be prepared for
> > > possible future changes, as suggested by Liam.
> > > 
> > > Reported-by: Guenter Roeck <[e-mail]>
> > > Closes: https://lore.kernel.org/[e-mail]
> > > Fixes: d0cf3dd47f0d ("damon: convert __damon_va_three_regions to use the VMA iterator")
> > > Suggested-by: Liam R. Howlett <[e-mail]>
> > > Signed-off-by: SeongJae Park <[e-mail]>
> > > ---
> > > Changes from v1
> > > (https://lore.kernel.org/[e-mail])
> > > - Keep lock usage and update the initialization flags (Liam)
> > 
> > Not sure I understand how this is better. Is the resulting rcu warning
> > considered to be irrelevant or a separate problem ?
> 
> I believe it's a separate potential problem (warning) that deserves a look.
> Meanwhile, this patch is fixing an issue that is definitely a bug, so better to
> fix right now, imho.
> 

Whenever you walk the tree, you need to hold the write lock or the rcu
read lock.  I believe that __damon_va_three_regions() needs to have the
rcu_read_lock()/ruc_read_unlock() around the for_each_vma() loop.

I sent a patch to the old thread that I suspect will fix it - but I
don't see the issue so it is hard to say for sure.

Thanks,
Liam

Guenter Roeck Sept. 4, 2024, 7:58 p.m. UTC | #4

On Wed, Sep 04, 2024 at 10:29:31AM -0700, SeongJae Park wrote:
> 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 clear when spinlock debugging is turned on, since it
> reports spinlock bad magic bug.
> 
> Fix the issue by excluding MT_FLAGS_LOCK_EXTERN from the maple tree
> initialization flags.  Note that we don't use empty flags to make it
> further similar to the usage of mm maple tree, and to be prepared for
> possible future changes, as suggested by Liam.
> 
> Reported-by: Guenter Roeck <[e-mail]>
> Closes: https://lore.kernel.org/[e-mail]
> Fixes: d0cf3dd47f0d ("damon: convert __damon_va_three_regions to use the VMA iterator")
> Suggested-by: Liam R. Howlett <[e-mail]>
> Signed-off-by: SeongJae Park <[e-mail]>

Tested-by: Guenter Roeck <[e-mail]>

> ---
> Changes from v1
> (https://lore.kernel.org/[e-mail])
> - Keep lock usage and update the initialization flags (Liam)
> - Fix a typo: s/celar/clear/ (Guenter)
> 
>  mm/damon/tests/vaddr-kunit.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/damon/tests/vaddr-kunit.h b/mm/damon/tests/vaddr-kunit.h
> index 83626483f82b..a339d117150f 100644
> --- 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_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");
>  
> -- 
> 2.39.2
>

Patch

diff --git a/mm/damon/tests/vaddr-kunit.h b/mm/damon/tests/vaddr-kunit.h
index 83626483f82b..a339d117150f 100644
--- 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_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");