FAILED: patch "[PATCH] mm/damon/core: remove unreachable damos charge_{target,addr}_from" failed to apply to 6.12-stable tree

The patch below does not apply to the 6.12-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.12.y
git checkout FETCH_HEAD
git cherry-pick -x 5695abc4e742b947f586bfb6a080a80ad9957e5d
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '[email protected]' --subject-prefix 'PATCH 6.12.y' 'HEAD^..'

Thanks,
Sasha

------------------ original commit in Linus's tree ------------------

From 5695abc4e742b947f586bfb6a080a80ad9957e5d Mon Sep 17 00:00:00 2001
From: SJ Park <[email protected]>
Date: Mon, 19 Oct 2026 19:39:56 -0700
Subject: [PATCH] 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.

---
 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)

2.53.0