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mm/damon/core: fix false positive in

PATCH Superseded Liew Rui Yan 2 participants 3 messages Sep 2

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Liew Rui Yan

author
#0 lore raw

PATCH v2 1/1 mm/damon/core: fix false positive in

When setting goal_tuner to 'temporal', if the goal is achieved, quota->esz_bp will be set to zero. In this case, damos_quota_is_full() will always return true, even if no regions have been tried to apply at all. This causes qt_exceeds to increase unexpectedly.

Fix it by:

- Returning false in damos_quota_is_full() when quota->charged_sz is zero.
- Adding an early exit in damon_do_apply_schemes() for schemes with zero esz.

Fixes c7ec7d5f6b3d ("mm/damon/core: handle <min_region_sz remaining quota as empty")
Cc <[email protected]> # v7.2.x
Signed-off-by Liew Rui Yan <[email protected]>

---

Changes from v1:
- v1: https://lore.kernel.org/damon/[email protected]
- Changed the check in damos_quota_is_full() from '!quota->esz' to '!quota->charged_sz', because the former is an upper bound, while the latter represents the actual quota usage.
- Added an early exit in damon_do_apply_schemes() to prevent a scheme with zero esz from being evaluated by damos_quota_is_full().
- Changed patch title, original title: always return false when esz is zero

---
 mm/damon/core.c | 5 +++++
 1 file changed, 5 insertions(+)
mm/damon/core.c +5 -0
index 8e3ded1a2cf6..de78ad56c8b3 100644
@@ -2560,6 +2560,8 @@ static bool damos_quota_is_full(struct damos_quota *quota,
 {
 	if (!damos_quota_is_set(quota))
 		return false;
+	if (!quota->charged_sz)
+		return false;
 	if (quota->charged_sz >= quota->esz)
 		return true;
 	/*

@@ -2666,6 +2668,9 @@ static void damon_do_apply_schemes(struct damon_ctx *c,
 		if (!s->wmarks.activated)
 			continue;
 
+		if (!quota->esz)
+			continue;
+
 		/* Check the quota */
 		if (damos_quota_is_full(quota, c->min_region_sz))
 			continue;
Signature · 1 line
2.55.0
AI review
#1 lore raw
Thank you for your contribution! Sashiko AI review found 2 potential issue(s) to consider: - [High] The newly added early exit `if (!quota->esz)` in `damon_do_apply_schemes()` unintentionally skips…

Thank you for your contribution! Sashiko AI review found 2 potential issue(s) to consider:
- [High] The newly added early exit if (!quota->esz) in damon_do_apply_schemes() unintentionally skips and completely breaks all DAMOS schemes that do not have quotas configured.
- [Medium] The new condition if (!quota->charged_sz) in damos_quota_is_full() incorrectly skips quota exhaustion checks when a configured quota is smaller than min_region_sz, violating per-region quota semantics and leading to statistic inflation and wasted CPU cycles. --

commit b189466ffcadd150b11edbb63a99f1fb0477b8b5
Author: Liew Rui Yan <[email protected]>

mm/damon/core: fix false positive in

This commit aims to fix a false positive in damos_quota_is_full() when goal_tuner is 'temporal' and the goal is achieved, causing quota->esz_bp to drop to zero. It adds checks for zero charged_sz and zero esz to prevent unexpectedly increasing qt_exceeds.

mm/damon/core.c static bool damos_quota_is_full(struct damos_quota *quota, · line 2659

Quoted 10 lines
mm/damon/core.c +2 -0
index df22621765911..f8c784df24600 100644
@@ -2655,6 +2655,8 @@ static bool damos_quota_is_full(struct damos_quota *quota,
 {
 	if (!damos_quota_is_set(quota))
 		return false;
+	if (!quota->charged_sz)
+		return false;

[Severity: Medium]
Does this unconditionally bypass the remaining quota check when no regions have been charged yet?

If a DAMOS scheme is evaluated with an effective quota (quota->esz) that is greater than 0 but strictly less than c->min_region_sz, returning false here skips the final check in this function:

	return quota->esz - quota->charged_sz < min_region_sz;

Could this cause the scheme to continue evaluating all targets and regions, only to calculate a zero size later in damos_apply_scheme() due to insufficient quota? This would artificially inflate s->stat.nr_tried for every region while wasting CPU cycles applying zero bytes.

Quoted 2 lines
mm/damon/core.c +0 -0
 	if (quota->charged_sz >= quota->esz)
 		return true;

[ ... ]

mm/damon/core.c static void damon_do_apply_schemes(struct damon_ctx *c, · line 2757

Quoted 7 lines
mm/damon/core.c +3 -0
@@ -2750,6 +2752,9 @@ static void damon_do_apply_schemes(struct damon_ctx *c,
 		if (!s->wmarks.activated)
 			continue;
 
+		if (!quota->esz)
+			continue;
+

[Severity: High]
Will this newly added check completely skip valid DAMOS schemes that simply do not have quotas configured?

When a user creates a scheme without quota limits (ms == 0, sz == 0, and no goals), damos_quota_init() initializes quota->esz to 0. Since damos_adjust_quota() leaves esz unmodified for un-set quotas, esz remains 0.

Because this check happens before damos_quota_is_full() (which normally handles non-quota schemes safely by returning false via damos_quota_is_set()), won't this cause damon_do_apply_schemes() to unconditionally skip all schemes that lack a quota?

Quoted 3 lines
mm/damon/core.c +0 -0
 		/* Check the quota */
 		if (damos_quota_is_full(quota, c->min_region_sz))
 			continue;
Signature · 1 line
Sashiko AI review · https://sashiko.dev/#/patchset/[email protected]?part=1
author
#2 lore raw

PATCH v2 1/1 mm/damon/core: fix false positive in

In reply to Liew Rui Yan
Quoted 58 lines · On Wed, 02 Sep 2026 16:18:25 +0800 Liew Rui Yan <[email protected]>…

When setting goal_tuner to 'temporal', if the goal is achieved, quota->esz_bp will be set to zero. In this case, damos_quota_is_full() will always return true, even if no regions have been tried to apply at all. This causes qt_exceeds to increase unexpectedly.

Fix it by:

- Returning false in damos_quota_is_full() when quota->charged_sz is zero.
- Adding an early exit in damon_do_apply_schemes() for schemes with zero esz.

Fixes c7ec7d5f6b3d ("mm/damon/core: handle <min_region_sz remaining quota as empty")
Cc <[email protected]> # v7.2.x
Signed-off-by Liew Rui Yan <[email protected]>

---

Changes from v1:
- v1: https://lore.kernel.org/damon/[email protected]
- Changed the check in damos_quota_is_full() from '!quota->esz' to '!quota->charged_sz', because the former is an upper bound, while the latter represents the actual quota usage.
- Added an early exit in damon_do_apply_schemes() to prevent a scheme with zero esz from being evaluated by damos_quota_is_full().
- Changed patch title, original title: always return false when esz is zero

---
 mm/damon/core.c | 5 +++++
 1 file changed, 5 insertions(+)
mm/damon/core.c +5 -0
index 8e3ded1a2cf6..de78ad56c8b3 100644
@@ -2560,6 +2560,8 @@ static bool damos_quota_is_full(struct damos_quota *quota,
 {
 	if (!damos_quota_is_set(quota))
 		return false;
+	if (!quota->charged_sz)
+		return false;
 	if (quota->charged_sz >= quota->esz)
 		return true;
 	/*

@@ -2666,6 +2668,9 @@ static void damon_do_apply_schemes(struct damon_ctx *c,
 		if (!s->wmarks.activated)
 			continue;
 
+		if (!quota->esz)
+			continue;
+
 		/* Check the quota */
 		if (damos_quota_is_full(quota, c->min_region_sz))
 			continue;
Signature · 1 line
2.55.0

Please ignore this patch. The title was truncated, so I've reposted it. If Sashiko provides a report in this thread that differs from the reposted version, I'll forward it as well.

Best regards,
Rui Yan