mm/damon/core: don't skip damos_adjust_quota() while esz is not zero

DAMOS could unexpectedly stop working when a user disables quota using the online parameters commit feature. Fix it by correcting a wrong quota unset check in damos_adjust_quota().

DAMON users could disable all quotas by unsetting time and size quotas, and removing all quota goals. The intention of disabling quotas would be making DAMOS run at full speed. When such quota disabled setup is detected, damos_adjust_quota() skips all its work. The skipped works include effective size quota (damos_quota->esz) updates and charged quota amount (damos_quota->charged_sz) resets. The intention is to avoid doing unnecessary work when quotas are disabled.

However, users could do the setup while effective size quota is non-zero, by doing the disabling with the online DAMON parameters commit feature. In this case, because the effective size quota exists, DAMOS will keep working with the quota until it is fully charged. After the effective quota is fully charged, the charged quota amount (damos_quota->charged_sz) cannot be reset because damos_adjust_quota() skips it. Then, DAMOS stops working until the quota is newly set or DAMON is entirely restarted.

The problem happens because damos_adjust_quota() assumes the user setup for disabling quota immediately disabled it. In reality, the quota is still working until the effective size quota is also updated to zero. Other logic for catching that uses damos_quota_is_set(), which understands the fact and therefore checks the effective size quota in addition to the user setup. Fix the issue by using damos_quota_is_set() in damos_adjust_quota() to determine if its works should be skipped.

The user impact is a non-deterministic and unexpected DAMOS stop behavior. That is, users would disable quotas using the online parameters commit feature, expecting DAMOS will run at full speed. However, depending on the timing, the setup can be updated while the effective size quota is non-zero. Due to the above mentioned internal mechanism, DAMOS stops working instead of running at full speed. It is unexpected behavior. It is also non-deterministic because sometimes the setup is done when the effective size quota is zero, depending on the timing. It doesn't cause critical issues like crashes or leaks. Users can simply set a reasonable quota again, or restart DAMON. But definitely it is an unexpected and non-deterministic behavior that makes it difficult to reliably use. Also investigating the root cause of the behavior would be quite difficult.

---
Changes from RFC v2
- RFC v2: https://lore.kernel.org/[email protected]
- Collect R-b: from Kunwu.
- Drop RFC tag.
- Rebase to latest mm-new.
Changes from RFC
- RFC: https://lore.kernel.org/[email protected]
- Split out from the cleanup series for taking hotfix path.
- Describe the real issue.

 mm/damon/core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index dd4317df42ff4..444c97c184027 100644
@@ -3348,7 +3348,7 @@ static void damos_adjust_quota(struct damon_ctx *c, struct damos *s)
 	unsigned long cumulated_sz, cached_esz;
 	unsigned int score, max_score = 0;
 
-	if (!quota->ms && !quota->sz && list_empty(&quota->goals))
+	if (!damos_quota_is_set(quota))
 		return;
 
 	/* First charge window */

base-commit: 8d713ff18bb27903ec1e19e54eecdda4168fe440

2.47.3