[RFC PATCH 00/10] mm/damon: add optional debugging-purpose sanity checks

[RFC PATCH 00/10] mm/damon: add optional debugging-purpose sanity checks

[SeongJae Park]

DAMON code has a few assumptions that can be critical if violated.
Validating the assumptions in code can be useful at finding such
critical bugs.  I was actually adding some such additional sanity checks
in my personal tree, and those were useful at finding bugs that I made
during the development of new patches.  We also found [1] sometimes the
assumptions are misunderstood.  The validation can work as good
documentation for such cases.

Add some of such debugging purpose sanity checks.  Because those
additional checks can impose more overhead, make those only optional via
new config, CONFIG_DAMON_DEBUG_SANITY, that is recommended for only
development and test setups.  And as recommended, enable it for DAMON
kunit tests and selftests.

[1] https://lore.kernel.org/20251231070029.79682-1-sj@kernel.org

SeongJae Park (10):
  mm/damon: add CONFIG_DAMON_DEBUG_SANITY
  mm/damon/core: add damon_new_region() debug_sanity check
  mm/damon/core: add damon_del_region() debug_sanity check
  mm/damon/core: add damon_nr_regions() debug_sanity check
  mm/damon/core: add damon_merge_two_regions() debug_sanity check
  mm/damon/core: add damon_merge_regions_of() debug_sanity check
  mm/damon/core: add damon_split_region_at() debug_sanity check
  mm/damon/core: add damon_reset_aggregated() debug_sanity check
  mm/damon/tests/.kunitconifg: enable DAMON_DEBUG_SANITY
  tools/testing/selftests/damon/config: enable DAMON_DEBUG_SANITY

 mm/damon/Kconfig                     |  11 +++
 mm/damon/core.c                      | 139 +++++++++++++++++++++++++++
 mm/damon/tests/.kunitconfig          |   3 +
 tools/testing/selftests/damon/config |   1 +
 4 files changed, 154 insertions(+)


base-commit: e059221c54896e36c8fc320029648e9013b382ea
-- 
2.47.3

[RFC PATCH 09/10] mm/damon/tests/.kunitconifg: enable DAMON_DEBUG_SANITY

[SeongJae Park]

CONFIG_DAMON_DEBUG_SANITY is recommended for DAMON development and test
setups.  Enable it on the default configurations for DAMON kunit test
run.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 mm/damon/tests/.kunitconfig | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/damon/tests/.kunitconfig b/mm/damon/tests/.kunitconfig
index 36a450f57b581..144d27e6ecc5c 100644
--- a/mm/damon/tests/.kunitconfig
+++ b/mm/damon/tests/.kunitconfig
@@ -13,3 +13,6 @@ CONFIG_DAMON_VADDR_KUNIT_TEST=y
 CONFIG_SYSFS=y
 CONFIG_DAMON_SYSFS=y
 CONFIG_DAMON_SYSFS_KUNIT_TEST=y
+
+# enable DAMON_DEBUG_SANITY to catch any bug
+CONFIG_DAMON_DEBUG_SANITY=y
-- 
2.47.3

[RFC PATCH 10/10] tools/testing/selftests/damon/config: enable DAMON_DEBUG_SANITY

[SeongJae Park]

CONFIG_DAMON_DEBUG_SANITY is recommended for DAMON development and test
setups.  Enable it on the build config for DAMON selftests.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 tools/testing/selftests/damon/config | 1 +
 1 file changed, 1 insertion(+)

diff --git a/tools/testing/selftests/damon/config b/tools/testing/selftests/damon/config
index a68a9fead5dca..6304adacb741c 100644
--- a/tools/testing/selftests/damon/config
+++ b/tools/testing/selftests/damon/config
@@ -4,3 +4,4 @@ CONFIG_DAMON_PADDR=y
 CONFIG_DAMON_VADDR=y
 CONFIG_DAMON_RECLAIM=y
 CONFIG_DAMON_LRU_SORT=y
+CONFIG_DAMON_DEBUG_SANITY=y
-- 
2.47.3

Re: [RFC PATCH 09/10] mm/damon/tests/.kunitconifg: enable DAMON_DEBUG_SANITY

[SeongJae Park]

On Sat, 21 Feb 2026 11:36:26 -0800 SeongJae Park <sj@kernel.org> wrote:

> CONFIG_DAMON_DEBUG_SANITY is recommended for DAMON development and test
> setups.  Enable it on the default configurations for DAMON kunit test
> run.
> 
> Signed-off-by: SeongJae Park <sj@kernel.org>
> ---
>  mm/damon/tests/.kunitconfig | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/mm/damon/tests/.kunitconfig b/mm/damon/tests/.kunitconfig
> index 36a450f57b581..144d27e6ecc5c 100644
> --- a/mm/damon/tests/.kunitconfig
> +++ b/mm/damon/tests/.kunitconfig
> @@ -13,3 +13,6 @@ CONFIG_DAMON_VADDR_KUNIT_TEST=y
>  CONFIG_SYSFS=y
>  CONFIG_DAMON_SYSFS=y
>  CONFIG_DAMON_SYSFS_KUNIT_TEST=y
> +
> +# enable DAMON_DEBUG_SANITY to catch any bug
> +CONFIG_DAMON_DEBUG_SANITY=y

And this makes kunit for DAMON fails, like below.

'''
$ ./tools/testing/kunit/kunit.py run --kunitconfig mm/damon/tests/
[11:57:20] Configuring KUnit Kernel ...
Regenerating .config ...
Populating config with:
$ make ARCH=um O=.kunit olddefconfig
[11:57:24] Building KUnit Kernel ...
Populating config with:
$ make ARCH=um O=.kunit olddefconfig
Building with:
$ make all compile_commands.json scripts_gdb ARCH=um O=.kunit --jobs=8
ERROR:root:In file included from ../include/asm-generic/bug.h:7,
                 from ./arch/um/include/generated/asm/bug.h:1,
                 from ../arch/x86/include/asm/alternative.h:9,
                 from ../arch/x86/um/asm/barrier.h:6,
                 from ../include/linux/list.h:11,
                 from ../arch/um/include/linux/time-internal.h:9,
                 from ../arch/x86/um/asm/processor.h:4,
                 from ../include/linux/sched.h:13,
                 from ../include/linux/cgroup.h:12,
                 from ../include/linux/memcontrol.h:13,
                 from ../include/linux/damon.h:11,
                 from ../mm/damon/core.c:10:
../mm/damon/core.c: In function ‘damon_verify_new_region’:
../include/linux/once_lite.h:28:50: error: expected expression before ‘)’ token
   28 |                 bool __ret_do_once = !!(condition);                     \
      |                                                  ^
../include/asm-generic/bug.h:185:9: note: in expansion of macro ‘DO_ONCE_LITE_IF’
  185 |         DO_ONCE_LITE_IF(condition, WARN, 1, format)
      |         ^~~~~~~~~~~~~~~
../mm/damon/core.c:127:9: note: in expansion of macro ‘WARN_ONCE’
  127 |         WARN_ONCE();
      |         ^~~~~~~~~
'''

I changed BUG() to WARN_ONCE() on the last moment of this patch posting, so I
missed this failure.  I will fix this on the next revision.


Thanks,
SJ

[...]

Re: [RFC PATCH 10/10] tools/testing/selftests/damon/config: enable DAMON_DEBUG_SANITY

[SeongJae Park]

It is more common to use 'selftests/damon' as the prefix of DAMON selftests
patches.  I will do so in the next revision.


Thanks,
SJ

[...]