[PATCH 6.12.y 5/5] mm/damon/core: remove unreachable damos charge_{target,addr}_from

[SJ Park]

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.

Link: https://lore.kernel.org/20261020023958.108276-1-sj@xxxxxxxxxx
Fixes: da87878010e5 ("mm/damon/sysfs: support online inputs update")
Signed-off-by: SJ Park <sj@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Closes: https://sashiko.dev/#/bug/linux-cd0c55fc-0cc8-45d8-9637-3334811c49b8
Cc: <stable@xxxxxxxxxxxxxxx> # 5.19.x
(cherry picked from commit 5695abc4e742b947f586bfb6a080a80ad9957e5d)
Signed-off-by: SJ Park <sj@xxxxxxxxxx>
---
 mm/damon/core.c | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index 046c07b86a6e7..badd0628e5eb2 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -1686,6 +1686,26 @@ static void damos_set_effective_quota(struct damos_quota *quota)
 	quota->esz = 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;
@@ -1714,6 +1734,8 @@ static void damos_adjust_quota(struct damon_ctx *c, struct damos *s)
 		damos_set_effective_quota(quota);
 	}
 
+	damos_fix_stale_charge_pointers(c, s);
+
 	if (!c->ops.get_scheme_score)
 		return;
 
-- 
2.47.3