mm/damon: fix misc bugs in kunit, quota goals and sysfs refresh_ms

damon_test_commit_quota_goal_for() is set to test committing a new psi goal on an existing psi goal. However, damon_test_commit_quota_goal() is mistakenly not covering the test case. Add the test case.

Fixes 99f89debafc5 ("mm/damon/tests/core-kunit: add damos_commit_quota_goal() test")

Cc <[email protected]> # 6.19.x

Signed-off-by SJ Park <[email protected]>

---
 mm/damon/tests/core-kunit.h | 7 +++++++
 1 file changed, 7 insertions(+)

mm/damon/tests/core-kunit.h +7 -0

index 7071ec277b007..f1e11548c771b 100644
@@ -793,6 +793,13 @@ static void damos_test_commit_quota_goal(struct kunit *test)
 		.last_psi_total = 456,
 	};
 
+	damos_test_commit_quota_goal_for(test, &dst,
+			&(struct damos_quota_goal) {
+			.metric = DAMOS_QUOTA_SOME_MEM_PSI_US,
+			.target_value = 234,
+			.current_value = 345,
+			.last_psi_total = 567,
+			});
 	damos_test_commit_quota_goal_for(test, &dst,
 			&(struct damos_quota_goal){
 			.metric = DAMOS_QUOTA_USER_INPUT,

Signature · 1 line

2.47.3

When DAMOS_QUOTA_SOME_MEM_PSI_US metric damos quota goal is set, the PSI delta for the feedback loop is calculated using damos_quota_goal->last_psi_total. However, it is initialized only after the first feedback loop. The first iteration of the loop uses the uninitialized value. As a result, the feedback loop can change the effective quota in an unexpected way at the first iteration.

The user impact of the issue is not big, because the issue impacts only the first iteration of the feedback loop. The feedback loop also has an internal cap of the quota adjustment. The wrong adjustment will soon be corrected over a few iterations. For this reason, doing no initialization at commit time was intentional. It is also explicitly commented. That said, nobody likes behaviors that are unexpected or difficult to be expected.

Check last_psi_total initialization and skip the tuning round when it is not initialized. For this, initialize last_psi_total with U64_MAX in the goal creation and the goal commit time. U64_MAX means the field is not initialized. The tuning round shows the value and adjusts it to guarantee the current quota is maintained for the round, and last_psi_total is correctly initialized on the next round.

Before this change, committing a new PSI goal on an existing PSI goal with goal-only DAMON sysfs command (commit_schemes_quota_goals) just worked. After this change, the tuning round right after the commit will be unnecessarily skipped, because last_psi_total is unconditionally marked as not initialized in the damos_commit_quota_goal_union(). This is an intended tradeoff for simplicity. Skipping just one round of tuning is no problem. Meanwhile it makes both the code and the behavior simple to understand.

Also update the quota goal commit unit test for changed last_psi_total setup behavior.

The issue was discovered [1] by Sashiko.

[1] https://lore.kernel.org/[email protected]

Fixes 2dbb60f789cb ("mm/damon/core: implement PSI metric DAMOS quota goal")

Cc <[email protected]> # 6.9.x

Signed-off-by SJ Park <[email protected]>

---
 mm/damon/core.c             | 13 +++++++++++--
 mm/damon/tests/core-kunit.h |  9 +++------
 2 files changed, 14 insertions(+), 8 deletions(-)

mm/damon/core.c +11 -2

index 0df785e72438f..20748b0a71026 100644
@@ -636,6 +636,8 @@ struct damos_quota_goal *damos_new_quota_goal(
 		return NULL;
 	goal->metric = metric;
 	goal->target_value = target_value;
+	if (metric == DAMOS_QUOTA_SOME_MEM_PSI_US)
+		goal->last_psi_total = U64_MAX;
 	INIT_LIST_HEAD(&goal->list);
 	return goal;
 }

@@ -1129,6 +1131,9 @@ static void damos_commit_quota_goal_union(
 		struct damos_quota_goal *dst, struct damos_quota_goal *src)
 {
 	switch (dst->metric) {
+	case DAMOS_QUOTA_SOME_MEM_PSI_US:
+		dst->last_psi_total = U64_MAX;
+		break;
 	case DAMOS_QUOTA_NODE_MEM_USED_BP:
 	case DAMOS_QUOTA_NODE_MEM_FREE_BP:
 		dst->nid = src->nid;

@@ -1150,7 +1155,6 @@ static void damos_commit_quota_goal(
 	dst->target_value = src->target_value;
 	if (dst->metric == DAMOS_QUOTA_USER_INPUT)
 		dst->current_value = src->current_value;
-	/* keep last_psi_total as is, since it will be updated in next cycle */
 	damos_commit_quota_goal_union(dst, src);
 }
 

@@ -3039,7 +3043,12 @@ static void damos_set_quota_goal_current_value(struct damon_ctx *c,
 		break;
 	case DAMOS_QUOTA_SOME_MEM_PSI_US:
 		now_psi_total = damos_get_some_mem_psi_total();
-		goal->current_value = now_psi_total - goal->last_psi_total;
+		/* uninitialized last_psi_total; make no effect this round */
+		if (goal->last_psi_total == U64_MAX)
+			goal->current_value = goal->target_value;
+		else
+			goal->current_value = now_psi_total -
+				goal->last_psi_total;
 		goal->last_psi_total = now_psi_total;
 		break;
 	case DAMOS_QUOTA_NODE_MEM_USED_BP:

mm/damon/tests/core-kunit.h +0 -4

index f1e11548c771b..af26b3d60957b 100644
@@ -757,19 +757,16 @@ static void damos_test_commit_quota_goal_for(struct kunit *test,
 		struct damos_quota_goal *dst,
 		struct damos_quota_goal *src)
 {
-	u64 dst_last_psi_total = 0;
-
-	if (dst->metric == DAMOS_QUOTA_SOME_MEM_PSI_US)
-		dst_last_psi_total = dst->last_psi_total;
 	damos_commit_quota_goal(dst, src);
 

mm/damon/tests/core-kunit.h +0 -0

 	case DAMOS_QUOTA_NODE_MEM_USED_BP:
 	case DAMOS_QUOTA_NODE_MEM_FREE_BP:
 		KUNIT_EXPECT_EQ(test, dst->nid, src->nid);

Signature · 1 line

2.47.3

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:

Pre-existing issues:
- [High] Inconsistent state handling in damon_commit_ctx() on memory allocation failure. --

commit c0fdc35d2fb6726e726ba0e857811cbaefc34918
Author: SJ Park <[email protected]>

mm/damon/core: copy nid for eligible_mem_bp damos quota goal commit

damos_commit_quota_goal_union() is not updating the ->nid union field when the goal metric is DAMOS_QUITA_NODE_ELIGIBLE_MEM_BP. Hence, if a DAMOS quota goal of the type is online committed in a way that it will reuse other quota goal's memory space, the new goal will work with a garbage nid value. As a result, the DAMOS scheme can show unexpected aggressiveness.

Quoted 2 lines

damos_commit_quota_goal_union() is not updating the ->nid union field when the goal metric is DAMOS_QUITA_NODE_ELIGIBLE_MEM_BP.

This isn't a bug, but there's a typo (DAMOS_QUITA_NODE_ELIGIBLE_MEM_BP) in the commit message.

Quoted 6 lines

mm/damon/core.c +0 -0

index 647ac77f573bb..a499d7866c507 100644

mm/damon/core.c static void damos_commit_quota_goal_union( · line 1151

Quoted 9 lines

mm/damon/core.c +3 -0

@@ -1143,6 +1143,9 @@ static void damos_commit_quota_goal_union(
 		dst->nid = src->nid;
 		dst->memcg_id = src->memcg_id;
 		break;
+	case DAMOS_QUOTA_NODE_ELIGIBLE_MEM_BP:
+		dst->nid = src->nid;
+		break;
 	default:
 		break;
 	}

[Severity: High]
This isn't a bug introduced by this patch, but is a pre-existing logic flaw in the core DAMON commit mechanism on memory allocation failure.

In mm/damon/core.c:damon_commit_ctx():

	err = __damon_commit_ctx(test_ctx, src);
	if (err)
		goto out;
	err = __damon_commit_ctx(dst, src);

If the second call to __damon_commit_ctx() fails with -ENOMEM, won't it leave the active dst context corrupted?

For example, an OOM condition can occur here in
mm/damon/core.c:damon_commit_schemes():

		new_scheme = damon_new_scheme(&src_scheme->pattern,
				src_scheme->action,
				src_scheme->apply_interval_us,
				&src_scheme->quota, &src_scheme->wmarks,
				NUMA_NO_NODE);
		if (!new_scheme)
			return -ENOMEM;

A failure here exits the commit process without rolling back prior scheme updates, leaving the active dst context partially updated.

The kernel handles this by setting dst->maybe_corrupted = true, which forces the kdamond monitoring thread to terminate gracefully but permanently.

Could this be triggered by a user-initiated online parameter update encountering an OOM condition during the second __damon_commit_ctx() invocation?

Does this violate the atomic nature of damon_commit_ctx() (which uses test_ctx precisely to avoid partial updates on the active context) and cause an unintended loss of the running context?

Signature · 2 lines

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/[email protected]?part=3

damos_commit_quota_goal_union() is not updating the ->nid union field when the goal metric is DAMOS_QUITA_NODE_ELIGIBLE_MEM_BP. Hence, if a

damos_commit_quota_goal_union() is not updating the ->nid union field when the goal metric is DAMOS_QUITA_NODE_ELIGIBLE_MEM_BP. Hence, if a DAMOS quota goal of the type is online committed in a way that it will reuse other quota goal's memory space, the new goal will work with a garbage nid value. As a result, the DAMOS scheme can show unexpected aggressiveness. Do the update.

The user impact is not catastrophic. No leak or crash happens. Doing the quota goal online commit that can reproduce the issue is expected to be not common. This issue was not found by real users but the AI review. That said, the issue can reliably be reproduced.

This issue was discovered [1] by Sashiko.

[1] https://lore.kkernel.org/[email protected]

Fixes 9138e27a3bc3 ("mm/damon: add node_eligible_mem_bp goal metric")

Cc <[email protected]> # 7.2.x

Signed-off-by SJ Park <[email protected]>

---
 mm/damon/core.c | 3 +++
 1 file changed, 3 insertions(+)

mm/damon/core.c +3 -0

index 20748b0a71026..53c3f97e3f126 100644
@@ -1143,6 +1143,9 @@ static void damos_commit_quota_goal_union(
 		dst->nid = src->nid;
 		dst->memcg_id = src->memcg_id;
 		break;
+	case DAMOS_QUOTA_NODE_ELIGIBLE_MEM_BP:
+		dst->nid = src->nid;
+		break;
 	default:
 		break;
 	}

Signature · 1 line

2.47.3

On Tue, 1 Sep 2026 17:27:22 -0700 SJ Park <[email protected]> wrote:

Quoted 3 lines

This issue was discovered [1] by Sashiko.

[1] https://lore.kkernel.org/[email protected]

Hi SJ,

The fix is consistent with the other node-based quota metrics. I also checked that damos_get_node_eligible_mem_bp() directly uses goal->nid when calculating the current value.

One minor typo: the [1] has an extra k (kkernel should be kernel).

Reviewed-by Kunwu Chan <[email protected]>

When 'refresh_ms' is set, DAMON sysfs interface periodically updates auto-tuned parameters and DAMOS stats. The timestamp for the next refresh is initialized when a DAMON context starts, and updated in its damon_call() callback function. That is, each DAMON context updates it. However, the timestamp is a global variable that is shared with all the contexts. When there are multiple DAMON contexts having different refresh_ms, the update frequency will be changed, depending on the order of the contexts. When there are multiple kdamonds, it will be even more chaotic. Fix the problem by having the timestamp per each context.

The user impact is not very critical. It does not leak, corrupt or crash. The update will not be faster or slower than the lowest and largest refresh_ms values of the contexts, respectively. The user can also manually ask the updates on demand using kdamond state commands. That said, clearly this is a bug and can easily be reproduced.

Fixes 9fd7bb5083d1 ("mm/damon/sysfs: change next_update_jiffies to a global variable")

Cc <[email protected]> # 6.18.x

Signed-off-by SJ Park <[email protected]>

---
 mm/damon/sysfs.c | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

mm/damon/sysfs.c +3 -4

index 3c81b4c91ac0d..dcb739ce0a729 100644
@@ -1788,6 +1788,7 @@ struct damon_sysfs_kdamond {
 	struct damon_sysfs_contexts *contexts;
 	struct damon_ctx *damon_ctx;
 	unsigned int refresh_ms;
+	unsigned long next_refresh_jiffies;
 };
 
 static struct damon_sysfs_kdamond *damon_sysfs_kdamond_alloc(void)

@@ -2213,17 +2214,15 @@ static struct damon_ctx *damon_sysfs_build_ctx(
 	return ctx;
 }
 
-static unsigned long damon_sysfs_next_update_jiffies;
-
 static int damon_sysfs_repeat_call_fn(void *data)
 {
 	struct damon_sysfs_kdamond *sysfs_kdamond = data;
 
 	if (!sysfs_kdamond->refresh_ms)
 		return 0;
-	if (time_before(jiffies, damon_sysfs_next_update_jiffies))
+	if (time_before(jiffies, sysfs_kdamond->next_refresh_jiffies))
 		return 0;
-	damon_sysfs_next_update_jiffies = jiffies +
+	sysfs_kdamond->next_refresh_jiffies = jiffies +
 		msecs_to_jiffies(sysfs_kdamond->refresh_ms);
 
 	if (!mutex_trylock(&damon_sysfs_lock))

mm/damon/sysfs.c +0 -0

 	repeat_call_control->fn = damon_sysfs_repeat_call_fn;
 	repeat_call_control->data = kdamond;

Signature · 1 line

2.47.3

DAMOS quota goals commit unit test is mistakenly not testing a test case that was designed to test. DAMOS quota goals and DAMON sysfs refresh_ms file have bugs that can produce non critical but still unexpected behaviors. Fix the bugs.

Patch 1 fixes the DAMOS quota goals commit unit test to cover a mistakenly uncovered case. Patches 2 and 3 fix the bugs in DAMOS PSI goal initialization and eligible_mem_bp online commit, respectively. Patch 4 fixes the bug in DAMON sysfs refresh_ms file handling.

Hi SJ,

Quoted 4 lines

Patch 1 fixes the DAMOS quota goals commit unit test to cover a mistakenly uncovered case. Patches 2 and 3 fix the bugs in DAMOS PSI goal initialization and eligible_mem_bp online commit, respectively. Patch 4 fixes the bug in DAMON sysfs refresh_ms file handling.

I reviewed and tested the integrated four-patch series in mm-new:

a20e38846efb..72dc8fdaae62

The integrated commits had stable patch-ids matching the posted patches. DAMON KUnit passed 42/42 tests. I also ran test-only eligible_mem_bp nid and two-kdamond refresh-isolation regression cases; they passed 42/42 and 43/43 tests, respectively. Both test-only changes were then removed and the worktree returned clean at the unchanged series tip.

These were functional UML KUnit tests. The fixes look good to me.

For the series:

Reviewed-by Lian Wang <[email protected]>

Tested-by Lian Wang <[email protected]>