[RFC PATCH 0/3] mm/damon: move zero damos quota target_value handling to the core layer

[RFC PATCH 0/3] mm/damon: move zero damos quota target_value handling to the core layer

[SJ Park]

Having zero DAMOS quota target value can cause division by zero.  DAMON
API callers are checking the target value parameters to avoid that.  It
is easy to make mistakes in some of the multiple API callers.  Move that
to the core layer.

Patch 1 adds the corner case handling into the core layer DAMON
parameters validation logic.  Patches 2 and 3 remove no more needed
DAMON API callers side handling of the corner case in DAMON_LRU_SORT and
DMON_SAMPLIE_MTIER, respectively.

SJ Park (3):
  mm/damon/core: error damos_commit_quota_goal() for zero target_value
  Revert "mm/damon/lru_sort: error out for >10000 active_mem_bp"
  Revert "samples/damon/mtier: error out for zero quota goal target
    values"

 mm/damon/core.c       | 22 ++++++++++++++++------
 mm/damon/lru_sort.c   |  2 --
 samples/damon/mtier.c |  3 ---
 3 files changed, 16 insertions(+), 11 deletions(-)


base-commit: bb9dee2c45b87e872d6b1d5f812ebafaa952b7c5
-- 
2.47.3

[RFC PATCH 1/3] mm/damon/core: error damos_commit_quota_goal() for zero target_value

[SJ Park]

If a DAMOS scheme has a damos_quota_goal of zero target_value,
damos_quota_goal() could trigger division-by-zero error.  Hence each
DAMON API callers should do the zero target_value validation.  It is
easy to make mistakes.  Actually such bugs in DAMON_LRU_SORT and
DAMON_SAMPLE_MTIER were found and fixed [1].

It is better to handle the corner case only once in the core layer,
instead of multiple places in all DAMON API callers.  One
straightforward option is using an alternative denominator for the
corner case in the damos_quota_goal().  However, the zero target_value
is meaningless.  In this case, the quota goal is always evaluated as
achieved or over-achieved.  The quota will only keep being reduced.

Simply avoid using zero target_value by adding a check in the core layer
DAMOS quota goal parameters validation/commit path,
damos_commit_quota_goal().  Update it to return an error in the case.
Also update its caller to propagate the error.

[1] https://lore.kernel.org/20260803134034.15217-1-sj@kernel.org

Signed-off-by: SJ Park <sj@kernel.org>
---
 mm/damon/core.c | 22 ++++++++++++++++------
 1 file changed, 16 insertions(+), 6 deletions(-)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index 5a92e4fac6d92..183d505c08e4e 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -1193,15 +1193,18 @@ static void damos_commit_quota_goal_union(
 	}
 }
 
-static void damos_commit_quota_goal(
+static int damos_commit_quota_goal(
 		struct damos_quota_goal *dst, struct damos_quota_goal *src)
 {
+	if (!src->target_value)
+		return  -EINVAL;
 	dst->metric = src->metric;
 	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);
+	return 0;
 }
 
 /**
@@ -1219,14 +1222,17 @@ static void damos_commit_quota_goal(
 int damos_commit_quota_goals(struct damos_quota *dst, struct damos_quota *src)
 {
 	struct damos_quota_goal *dst_goal, *next, *src_goal, *new_goal;
-	int i = 0, j = 0;
+	int i = 0, j = 0, err;
 
 	damos_for_each_quota_goal_safe(dst_goal, next, dst) {
 		src_goal = damos_nth_quota_goal(i++, src);
-		if (src_goal)
-			damos_commit_quota_goal(dst_goal, src_goal);
-		else
+		if (src_goal) {
+			err = damos_commit_quota_goal(dst_goal, src_goal);
+			if (err)
+				return err;
+		} else {
 			damos_destroy_quota_goal(dst_goal);
+		}
 	}
 	damos_for_each_quota_goal_safe(src_goal, next, src) {
 		if (j++ < i)
@@ -1235,7 +1241,11 @@ int damos_commit_quota_goals(struct damos_quota *dst, struct damos_quota *src)
 				src_goal->metric, src_goal->target_value);
 		if (!new_goal)
 			return -ENOMEM;
-		damos_commit_quota_goal(new_goal, src_goal);
+		err = damos_commit_quota_goal(new_goal, src_goal);
+		if (err) {
+			damos_free_quota_goal(new_goal);
+			return err;
+		}
 		damos_add_quota_goal(dst, new_goal);
 	}
 	return 0;
-- 
2.47.3

[RFC PATCH 2/3] Revert "mm/damon/lru_sort: error out for >10000 active_mem_bp"

[SJ Park]

This reverts commit 06befa61c427e74319781e6f35a364cfc32dbae8.

The commit was made to avoid zero damos quota goal target value, because
it can trigger division-by-zero.  Now the core layer handles the corner
case.  It returns an error for any attempt setting the aero
target_value.  The corner case handling in DAMON_LRU_SORT is hence no
more needed.  Remove it.

Note that this slightly changes the user behavior.  It still disallows
active_mem_bp of 10,002.  But now it allows other >10,000 active_mem_bp
values. Setting >10,000 active_mem_bp makes not much sense.  But it
doesn't cause critical problems such as memory leak or crash, either.
Arguably that doesn't deserve additional code complexity.  Just allow
it.

Signed-off-by: SJ Park <sj@kernel.org>
---
 mm/damon/lru_sort.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/mm/damon/lru_sort.c b/mm/damon/lru_sort.c
index f25ee7326e87c..ad8e86dd3a93e 100644
--- a/mm/damon/lru_sort.c
+++ b/mm/damon/lru_sort.c
@@ -233,8 +233,6 @@ static int damon_lru_sort_add_quota_goals(struct damos *hot_scheme,
 
 	if (!active_mem_bp)
 		return 0;
-	if (10000 < active_mem_bp)
-		return -EINVAL;
 	goal = damos_new_quota_goal(DAMOS_QUOTA_ACTIVE_MEM_BP, active_mem_bp);
 	if (!goal)
 		return -ENOMEM;
-- 
2.47.3

[RFC PATCH 3/3] Revert "samples/damon/mtier: error out for zero quota goal target values"

[SJ Park]

This reverts commit a16fd3ad9d89b05475864da97327870464611736.

The commit was made to avoid zero damos quota goal target value, because
it can trigger division-by-zero.  Now the core layer handles the corner
case.  It returns an error for any attempt setting the aero
target_value.  The corner case handling in DAMON_SAMPLE_MTIER is hence
no more needed.  Remove it.

Signed-off-by: SJ Park <sj@kernel.org>
---
 samples/damon/mtier.c | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/samples/damon/mtier.c b/samples/damon/mtier.c
index bea45c87cc9be..27dc88bdf7a0e 100644
--- a/samples/damon/mtier.c
+++ b/samples/damon/mtier.c
@@ -161,9 +161,6 @@ static struct damon_ctx *damon_sample_mtier_build_ctx(bool promote)
 	if (!scheme)
 		goto free_out;
 	damon_set_schemes(ctx, &scheme, 1);
-	/* zero target value causes division by zero in damos_quota_store() */
-	if (!node0_mem_used_bp || !node0_mem_free_bp)
-		goto free_out;
 	quota_goal = damos_new_quota_goal(
 			promote ? DAMOS_QUOTA_NODE_MEM_USED_BP :
 			DAMOS_QUOTA_NODE_MEM_FREE_BP,
-- 
2.47.3