mm/damon/core: remove unreachable damos charge_{target,addr}_from

damos_quota->charge_{target,addr}_from saves the target and region the scheme's quota limit is reached. damos_skip_charged_region() skips the target and regions before the point. The target that was assigned to charge_target_from could be dynamically removed from the context by the runtime parameters update (aka commit) feature. In this case, damos_skip_charged_region() cannot find if the point is passed or not, so the scheme is effectively disabled.

Similarly, the user could also remove all the regions of the target using the runtime parameters update feature. It would be uncommon to do that, but it is a supported feature. Because the skip pointer detection logic (damos_skip_charged_region()) works for each region of targets, this result in the logic is entirely skipped and the scheme is effectively disabled again.

The user impact is that the DAMOS scheme could suddenly stop working under certain uses of the runtime parameters update feature. It could surprise users. No crash or leak happens though. Users could also simply work around by re-installing the scheme.

Catch the corner cases in damos_adjust_quota() and reset the pointers.

---
Changes from RFC
- RFC: https://lore.kernel.org/[email protected]
- Drop RFC tag.

 mm/damon/core.c | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

index 479169dc9ac75..eb1fcdabc0075 100644
@@ -3383,6 +3383,26 @@ static void damos_trace_esz(struct damon_ctx *c, struct damos *s,
 	trace_damos_esz(cidx, sidx, quota->esz);
 }
 
+static void damos_fix_stale_charge_pointers(struct damon_ctx *c,
+		struct damos *s)
+{
+	struct damos_quota *quota;
+	struct damon_target *t;
+
+	quota = &s->quota;
+	if (!quota->charge_target_from)
+		return;
+	damon_for_each_target(t, c) {
+		if (t == quota->charge_target_from) {
+			if (damon_nr_regions(t) > 0)
+				return;
+			break;
+		}
+	}
+	quota->charge_target_from = NULL;
+	quota->charge_addr_from = 0;
+}
+
 static void damos_adjust_quota(struct damon_ctx *c, struct damos *s)
 {
 	struct damos_quota *quota = &s->quota;

@@ -3418,6 +3438,8 @@ static void damos_adjust_quota(struct damon_ctx *c, struct damos *s)
 			damos_trace_esz(c, s, quota);
 	}
 
+	damos_fix_stale_charge_pointers(c, s);
+
 	if (damos_quota_is_full(quota, c->min_region_sz))
 		return;
 	if (!c->ops.get_scheme_score)

base-commit: e558ba2a16a47baf6b8e76bb11e92eb92a491e3a

2.47.3

Hi SJ,

The patch looks good to me. I tested DAMON_RECLAIM stop/start after filling the quota on x86-64 KVM and everything goes well :)

Thanks,
Aveline Noir

Appreciate reviews and tests to this and other patches, Aveline!

Thanks,
SJ

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