[RFC PATCH v1.1 0/3] mm/damon: fix wrong behaviors in DAMOS quota goals and sysfs refresh_ms

[RFC PATCH v1.1 0/3] mm/damon: fix wrong behaviors in DAMOS quota goals and sysfs refresh_ms

[SJ Park]

DAMOS quota goals and DAMON sysfs refresh_ms file have bugs that can
produce non critical but still unexpected behaviors.  Fix the two bugs.

Patches 1 and 2 fix the bugs in DAMOS PSI goal initialization and
eligible_mem_bp online commit, respectively.  Patch 3 fixes the bug in
DAMON sysfs refresh_ms file handling.

Changes from RFC
- RFC: https://lore.kernel.org/20260827042920.93580-1-sj@kernel.org
  (Mistakenly missed RFC tag)
- Fix build error.
- Add eligible_mem_bp nid commit fix.

SJ Park (3):
  mm/damon/core: initialize damos_quota_goal->last_psi_total
  mm/damon/core: copy nid for eligible_mem_bp damos quota goal commit
  mm/damon/sysfs: set next refresh jiffies per sysfs context

 mm/damon/core.c  | 21 ++++++++++++++++++++-
 mm/damon/sysfs.c | 11 +++++------
 2 files changed, 25 insertions(+), 7 deletions(-)


base-commit: 9e11dd3caaf0ac70acdc0f568265a7fa5085ef42
-- 
2.47.3

[RFC PATCH v1.1 1/3] mm/damon/core: initialize damos_quota_goal->last_psi_total

[SJ Park]

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.  Fixing it is also simple and not expensive.

Initialize the field at the beginning of kdamond and each commit of
SOME_MEM_PSI_US type quota goals.

The issue was discovered [1] by Sashiko.

[1] https://lore.kernel.org/20260718005316.89585-1-sj@kernel.org

Fixes: 2dbb60f789cb ("mm/damon/core: implement PSI metric DAMOS quota goal")
Cc: <stable@vger.kernel.org> # 6.9.x
Signed-off-by: SJ Park <sj@kernel.org>
---
 mm/damon/core.c | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index f8dddbff74a77..e5b6b1c6bb7b2 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -1118,10 +1118,15 @@ static struct damos_quota_goal *damos_nth_quota_goal(
 	return NULL;
 }
 
+static u64 damos_get_some_mem_psi_total(void);
+
 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 = damos_get_some_mem_psi_total();
+		break;
 	case DAMOS_QUOTA_NODE_MEM_USED_BP:
 	case DAMOS_QUOTA_NODE_MEM_FREE_BP:
 		dst->nid = src->nid;
@@ -1143,7 +1148,6 @@ 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);
 }
 
@@ -3741,6 +3745,17 @@ static int kdamond_wait_activation(struct damon_ctx *ctx)
 	return -EBUSY;
 }
 
+static void damos_init_quota_goal_last_psi(struct damos *s)
+{
+	struct damos_quota_goal *goal;
+
+	damos_for_each_quota_goal(goal, &s->quota) {
+		if (goal->metric != DAMOS_QUOTA_SOME_MEM_PSI_US)
+			continue;
+		goal->last_psi_total = damos_get_some_mem_psi_total();
+	}
+}
+
 static void kdamond_init_ctx(struct damon_ctx *ctx)
 {
 	unsigned long sample_interval = ctx->attrs.sample_interval ?
@@ -3757,6 +3772,7 @@ static void kdamond_init_ctx(struct damon_ctx *ctx)
 	damon_for_each_scheme(scheme, ctx) {
 		damos_set_next_apply_sis(scheme, ctx);
 		damos_set_filters_default_reject(scheme);
+		damos_init_quota_goal_last_psi(scheme);
 	}
 }
 
-- 
2.47.3

[RFC PATCH v1.1 2/3] mm/damon/core: copy nid for eligible_mem_bp damos quota goal commit

[SJ Park]

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/20260827045035.94611-1-sj@kernel.org

Fixes: 9138e27a3bc3 ("mm/damon: add node_eligible_mem_bp goal metric")
Cc: <stable@vger.kernel.org> # 7.2.x
Signed-off-by: SJ Park <sj@kernel.org>
---
 mm/damon/core.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index e5b6b1c6bb7b2..11b20a4a726f9 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -1136,6 +1136,9 @@ 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;
 	}
-- 
2.47.3

[RFC PATCH v1.1 3/3] mm/damon/sysfs: set next refresh jiffies per sysfs context

[SJ Park]

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: <stable@vger.kernel.org> # 6.18.x
Signed-off-by: SJ Park <sj@kernel.org>
---
 mm/damon/sysfs.c | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/mm/damon/sysfs.c b/mm/damon/sysfs.c
index e3858ffab4b22..f05b256c90ee7 100644
--- a/mm/damon/sysfs.c
+++ b/mm/damon/sysfs.c
@@ -1789,6 +1789,7 @@ 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)
@@ -2215,17 +2216,15 @@ 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))
@@ -2273,8 +2272,8 @@ static int damon_sysfs_turn_damon_on(struct damon_sysfs_kdamond *kdamond)
 	}
 	kdamond->damon_ctx = ctx;
 
-	damon_sysfs_next_update_jiffies =
-		jiffies + msecs_to_jiffies(kdamond->refresh_ms);
+	kdamond->next_refresh_jiffies = jiffies +
+		msecs_to_jiffies(kdamond->refresh_ms);
 
 	repeat_call_control->fn = damon_sysfs_repeat_call_fn;
 	repeat_call_control->data = kdamond;
-- 
2.47.3