[RFC PATCH v2 00/10] mm/damon: add optional debugging-purpose sanity checks

[RFC PATCH v2 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.

Note that the verification only WARN_ON() for each of the insanity.  The
developer or tester may better to set panic_on_oops together, like
damon-tests/corr did [2].

[1] https://lore.kernel.org/20251231070029.79682-1-sj@kernel.org
[2] https://github.com/damonitor/damon-tests/commit/a80fbee55e272f151b4e5809ee85898aea33e6ff

Changes from RFC v1
(https://lore.kernel.org/20260221193629.19332-1-sj@kernel.org)
- Fix WARN_ON() failure on kunit UML build
- Fix Wformat-zero-length issue
  (https://lore.kernel.org/oe-kbuild-all/202602221352.odZW8x1R-lkp@intel.com/)
- Rebase to latest mm-new
- Make warning message less verbose
- Use WARN_ONCE() condition
- Do the verification asap
- Minor cleanups

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
  selftests/damon/config: enable DAMON_DEBUG_SANITY

 mm/damon/Kconfig                     |  11 +++
 mm/damon/core.c                      | 109 +++++++++++++++++++++++++++
 mm/damon/tests/.kunitconfig          |   3 +
 tools/testing/selftests/damon/config |   1 +
 4 files changed, 124 insertions(+)


base-commit: b511f483f5fc7e85ad60679181503f16937feb6e
-- 
2.47.3

[RFC PATCH v2 01/10] mm/damon: add CONFIG_DAMON_DEBUG_SANITY

[SeongJae Park]

Add a new build config that will enable additional DAMON sanity checks.
It is recommended to be enabled on only development and test setups,
since it can impose additional overhead.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 mm/damon/Kconfig | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/mm/damon/Kconfig b/mm/damon/Kconfig
index 8c868f7035fce..34631a44cdecb 100644
--- a/mm/damon/Kconfig
+++ b/mm/damon/Kconfig
@@ -12,6 +12,17 @@ config DAMON
 	  See https://www.kernel.org/doc/html/latest/mm/damon/index.html for
 	  more information.
 
+config DAMON_DEBUG_SANITY
+	bool "Check sanity of DAMON code"
+	depends on DAMON
+	help
+	  This enables additional DAMON debugging-purpose sanity checks in
+	  DAMON code.  This can be useful for finding bugs, but impose
+	  additional overhead.  This is therefore recommended to be enabled on
+	  only development and test setups.
+
+	  If unsure, say N.
+
 config DAMON_KUNIT_TEST
 	bool "Test for damon" if !KUNIT_ALL_TESTS
 	depends on DAMON && KUNIT=y
-- 
2.47.3

[RFC PATCH v2 02/10] mm/damon/core: add damon_new_region() debug_sanity check

[SeongJae Park]

damon_new_region() is supposed to be called with only valid address
range arguments.  Do the check under DAMON_DEBUG_SANITY.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 mm/damon/core.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index f1a97e85824ac..0c1353164ec81 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -109,6 +109,17 @@ int damon_select_ops(struct damon_ctx *ctx, enum damon_ops_id id)
 	return err;
 }
 
+#ifdef CONFIG_DAMON_DEBUG_SANITY
+static void damon_verify_new_region(unsigned long start, unsigned long end)
+{
+	WARN_ONCE(start >= end, "start %lu >= end %lu\n", start, end);
+}
+#else
+static void damon_verify_new_region(unsigned long start, unsigned long end)
+{
+}
+#endif
+
 /*
  * Construct a damon_region struct
  *
@@ -118,6 +129,7 @@ struct damon_region *damon_new_region(unsigned long start, unsigned long end)
 {
 	struct damon_region *region;
 
+	damon_verify_new_region(start, end);
 	region = kmem_cache_alloc(damon_region_cache, GFP_KERNEL);
 	if (!region)
 		return NULL;
-- 
2.47.3

[RFC PATCH v2 03/10] mm/damon/core: add damon_del_region() debug_sanity check

[SeongJae Park]

damon_del_region() should be called for targets that have one or more
regions.  Add a sanity check for that under CONFIG_DAMON_DEBUG_SANITY.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 mm/damon/core.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index 0c1353164ec81..92eac5eb96026 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -152,8 +152,21 @@ void damon_add_region(struct damon_region *r, struct damon_target *t)
 	t->nr_regions++;
 }
 
+#ifdef CONFIG_DAMON_DEBUG_SANITY
+static void damon_verify_del_region(struct damon_target *t)
+{
+	WARN_ONCE(t->nr_regions == 0, "t->nr_regions == 0\n");
+}
+#else
+static void damon_verify_del_region(struct damon_target *t)
+{
+}
+#endif
+
 static void damon_del_region(struct damon_region *r, struct damon_target *t)
 {
+	damon_verify_del_region(t);
+
 	list_del(&r->list);
 	t->nr_regions--;
 }
-- 
2.47.3

[RFC PATCH v2 04/10] mm/damon/core: add damon_nr_regions() debug_sanity check

[SeongJae Park]

damon_target->nr_regions is introduced to get the number quickly without
having to iterate regions always.  Add a sanity check for that under
CONFIG_DAMON_DEBUG_SANITY.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 mm/damon/core.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index 92eac5eb96026..2b5aed89d449d 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -545,8 +545,27 @@ void damon_destroy_target(struct damon_target *t, struct damon_ctx *ctx)
 	damon_free_target(t);
 }
 
+#ifdef CONFIG_DAMON_DEBUG_SANITY
+static void damon_verify_nr_regions(struct damon_target *t)
+{
+	struct damon_region *r;
+	unsigned int count = 0;
+
+	damon_for_each_region(r, t)
+		count++;
+	WARN_ONCE(count != t->nr_regions, "t->nr_regions (%u) != count (%u)\n",
+			t->nr_regions, count);
+}
+#else
+static void damon_verify_nr_regions(struct damon_target *t)
+{
+}
+#endif
+
 unsigned int damon_nr_regions(struct damon_target *t)
 {
+	damon_verify_nr_regions(t);
+
 	return t->nr_regions;
 }
 
-- 
2.47.3

[RFC PATCH v2 05/10] mm/damon/core: add damon_merge_two_regions() debug_sanity check

[SeongJae Park]

A data corruption could cause damon_merge_two_regions() creating zero
length DAMON regions.  Add a sanity check for that under
CONFIG_DAMON_DEBUG_SANITY.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 mm/damon/core.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index 2b5aed89d449d..8a3738a17cdd9 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -2473,6 +2473,21 @@ static void kdamond_apply_schemes(struct damon_ctx *c)
 	mutex_unlock(&c->walk_control_lock);
 }
 
+#ifdef CONFIG_DAMON_DEBUG_SANITY
+static void damon_verify_merge_two_regions(
+		struct damon_region *l, struct damon_region *r)
+{
+	/* damon_merge_two_regions() may created incorrect left region */
+	WARN_ONCE(l->ar.start >= l->ar.end, "l: %lu-%lu, r: %lu-%lu\n",
+			l->ar.start, l->ar.end, r->ar.start, r->ar.end);
+}
+#else
+static void damon_verify_merge_two_regions(
+		struct damon_region *l, struct damon_region *r)
+{
+}
+#endif
+
 /*
  * Merge two adjacent regions into one region
  */
@@ -2486,6 +2501,7 @@ static void damon_merge_two_regions(struct damon_target *t,
 	l->nr_accesses_bp = l->nr_accesses * 10000;
 	l->age = (l->age * sz_l + r->age * sz_r) / (sz_l + sz_r);
 	l->ar.end = r->ar.end;
+	damon_verify_merge_two_regions(l, r);
 	damon_destroy_region(r, t);
 }
 
-- 
2.47.3

[RFC PATCH v2 06/10] mm/damon/core: add damon_merge_regions_of() debug_sanity check

[SeongJae Park]

damon_merge_regions_of() should be called only after aggregation is
finished and therefore each region's nr_accesses and nr_accesses_bp
match.  There were bugs that broke the assumption, during development of
online DAMON parameter updates and monitoring results handling changes.
Add a sanity check for that under CONFIG_DAMON_DEBUG_SANITY.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 mm/damon/core.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index 8a3738a17cdd9..93d1577b1ae07 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -2505,6 +2505,20 @@ static void damon_merge_two_regions(struct damon_target *t,
 	damon_destroy_region(r, t);
 }
 
+#ifdef CONFIG_DAMON_DEBUG_SANITY
+static void damon_verify_merge_regions_of(struct damon_region *r)
+{
+	WARN_ONCE(r->nr_accesses != r->nr_accesses_bp / 10000,
+			"nr_accesses (%u) != nr_accesses_bp (%u)\n",
+			r->nr_accesses, r->nr_accesses_bp);
+}
+#else
+static void damon_verify_merge_regions_of(struct damon_region *r)
+{
+}
+#endif
+
+
 /*
  * Merge adjacent regions having similar access frequencies
  *
@@ -2518,6 +2532,7 @@ static void damon_merge_regions_of(struct damon_target *t, unsigned int thres,
 	struct damon_region *r, *prev = NULL, *next;
 
 	damon_for_each_region_safe(r, next, t) {
+		damon_verify_merge_regions_of(r);
 		if (abs(r->nr_accesses - r->last_nr_accesses) > thres)
 			r->age = 0;
 		else if ((r->nr_accesses == 0) != (r->last_nr_accesses == 0))
-- 
2.47.3

[RFC PATCH v2 07/10] mm/damon/core: add damon_split_region_at() debug_sanity check

[SeongJae Park]

damon_split_region_at() should be called with the correct address to
split on.  Add a sanity check for that under CONFIG_DAMON_DEBUG_SANITY.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 mm/damon/core.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index 93d1577b1ae07..be2300cf3aeae 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -2586,6 +2586,21 @@ static void kdamond_merge_regions(struct damon_ctx *c, unsigned int threshold,
 			threshold / 2 < max_thres);
 }
 
+#ifdef CONFIG_DAMON_DEBUG_SANITY
+static void damon_verify_split_region_at(struct damon_region *r,
+		unsigned long sz_r)
+{
+	WARN_ONCE(sz_r == 0 || sz_r >= damon_sz_region(r),
+			"sz_r: %lu r: %lu-%lu (%lu)\n",
+			sz_r, r->ar.start, r->ar.end, damon_sz_region(r));
+}
+#else
+static void damon_verify_split_region_at(struct damon_region *r,
+		unsigned long sz_r)
+{
+}
+#endif
+
 /*
  * Split a region in two
  *
@@ -2597,6 +2612,7 @@ static void damon_split_region_at(struct damon_target *t,
 {
 	struct damon_region *new;
 
+	damon_verify_split_region_at(r, sz_r);
 	new = damon_new_region(r->ar.start + sz_r, r->ar.end);
 	if (!new)
 		return;
-- 
2.47.3

[RFC PATCH v2 08/10] mm/damon/core: add damon_reset_aggregated() debug_sanity check

[SeongJae Park]

At time of damon_reset_aggregated(), aggregation of the interval should
be completed, and hence nr_accesses and nr_accesses_bp should match.  I
found a few bugs caused it to be broken in the past, from online
parameters update and complicated nr_accesses handling changes.  Add a
sanity check for that under CONFIG_DAMON_DEBUG_SANITY.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 mm/damon/core.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index be2300cf3aeae..e8c44541754f7 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -1666,6 +1666,23 @@ static void damon_warn_fix_nr_accesses_corruption(struct damon_region *r)
 	r->nr_accesses_bp = r->nr_accesses * 10000;
 }
 
+#ifdef CONFIG_DAMON_DEBUG_SANITY
+static void damon_verify_reset_aggregated(struct damon_region *r,
+		struct damon_ctx *c)
+{
+	WARN_ONCE(r->nr_accesses_bp != r->last_nr_accesses * 10000,
+			"nr_accesses_bp %u last_nr_accesses %u sis %lu %lu\n",
+			r->nr_accesses_bp, r->last_nr_accesses,
+			c->passed_sample_intervals, c->next_aggregation_sis);
+}
+#else
+static void damon_verify_reset_aggregated(struct damon_region *r,
+		struct damon_ctx *c)
+{
+}
+#endif
+
+
 /*
  * Reset the aggregated monitoring results ('nr_accesses' of each region).
  */
@@ -1682,6 +1699,7 @@ static void kdamond_reset_aggregated(struct damon_ctx *c)
 			damon_warn_fix_nr_accesses_corruption(r);
 			r->last_nr_accesses = r->nr_accesses;
 			r->nr_accesses = 0;
+			damon_verify_reset_aggregated(r, c);
 		}
 		ti++;
 	}
-- 
2.47.3

[RFC PATCH v2 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 v2 10/10] 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