mm/damon/core: error damos_commit_quota_goal() for zero target_value

Patch series "mm/damon: move zero damos quota target_value handling to the core layer".

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.

This patch (of 3):

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.

Notes for stable kernel porting. The upstream commit doesn't have
Fixes: and therefore not ported to stable kernels. It turned out it fixes commit 8b549a4fd3c5 ("mm/damon/sysfs-schemes: commit damos quota goals user input to DAMOS") which was merged into v6.8. The user impact is a race condition that can result in divide-by-zero. Hence backport. Reproducing the issue requires sysfs write permission, certain DAMON ABI usages that are supposed to be uncommon, and a rare race timing, though. The issue was discovered [2] by Sashiko as a part [3] of its bug report.

Also, the upstream commit on this stable kernel can fix the issue for parameters commit paths but DAMON start path. That's because DAMON start path does not invoke damon_commit_quota_goals(). To fix the path with minimum change, add a new small validation of quota goal target value before damon_start() call.

[2] https://sashiko.dev/#/bug/linux-7f814c9a-986f-4103-a55a-ef9537dcf62b
[3] https://lore.kernel.org/[email protected]

---
Changes from RFC v2
- RFC v2: https://lore.kernel.org/[email protected]
- Put target pid reference count in damon_start() path -EINVAL case. Changes from RFC
- RFC: https://lore.kernel.org/[email protected]
- Fix a typo and macro expansion build error for quota goal iteration.
- Destroy ctx in the before-damon_start() error path.

 mm/damon/core.c  | 18 +++++++++++++-----
 mm/damon/sysfs.c | 19 +++++++++++++++++++
 2 files changed, 32 insertions(+), 5 deletions(-)

index 81d44e41de210..ed32855c16ed8 100644
@@ -698,14 +698,17 @@ static struct damos_quota_goal *damos_nth_quota_goal(
 	return NULL;
 }
 
-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 */
+	return 0;
 }
 
 /**

@@ -723,18 +726,23 @@ 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)
 			continue;
+		if (!src_goal->target_value)
+			return -EINVAL;
 		new_goal = damos_new_quota_goal(
 				src_goal->metric, src_goal->target_value);
 		if (!new_goal)

index 5aa23c6545cb8..ad9fa8a86c9ef 100644
@@ -1497,6 +1497,20 @@ static struct damon_ctx *damon_sysfs_build_ctx(
 	return ctx;
 }
 
+static bool damon_sysfs_valid_start_ctx(struct damon_ctx *ctx)
+{
+	struct damos *s;
+	struct damos_quota_goal *g;
+
+	damon_for_each_scheme(s, ctx) {
+		damos_for_each_quota_goal(g, (&s->quota)) {
+			if (!g->target_value)
+				return false;
+		}
+	}
+	return true;
+}
+
 static int damon_sysfs_turn_damon_on(struct damon_sysfs_kdamond *kdamond)
 {
 	struct damon_ctx *ctx;

@@ -1517,6 +1531,11 @@ static int damon_sysfs_turn_damon_on(struct damon_sysfs_kdamond *kdamond)
 	ctx = damon_sysfs_build_ctx(kdamond->contexts->contexts_arr[0]);
 	if (IS_ERR(ctx))
 		return PTR_ERR(ctx);
+	if (!damon_sysfs_valid_start_ctx(ctx)) {
+		damon_sysfs_destroy_targets(ctx);
+		damon_destroy_ctx(ctx);
+		return -EINVAL;
+	}
 	err = damon_start(&ctx, 1, false);
 	if (err) {
 		damon_destroy_ctx(ctx);

base-commit: 85d82e0d7c2dfd3e75d34f35b7ebbf41f45c9c4e

2.47.3