[PATCH v2.1] mm/damon/core: fix false positive in damos_quota_is_full() when esz is zero

[Liew Rui Yan <aethernet65535@gmail.com>]

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: <stable@vger.kernel.org> # v7.2.x
Signed-off-by: Liew Rui Yan <aethernet65535@gmail.com>
---

Changes from v2:
- v2: https://lore.kernel.org/damon/20260902081825.18931-1-aethernet65535@gmail.com
- Changed patch title, v2 title is truncated.

Changes from v1:
- v1: https://lore.kernel.org/damon/20260901105925.106186-1-aethernet65535@gmail.com
- 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(+)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index 8e3ded1a2cf6..de78ad56c8b3 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -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;
-- 
2.55.0