[RFC PATCH v1.3 3/4] mm/damon/core: copy nid for eligible_mem_bp damos quota goal commit

[SJ Park <sj@kernel.org>]

damos_commit_quota_goal_union() is not updating the ->nid union field
when the goal metric is DAMOS_QUITA_NODE_ELIGIBLE_MEM_BP.  Hence, if a
DAMOS quota goal of the type is online committed in a way that it will
reuse other quota goal's memory space, the new goal will work with a
garbage nid value.  As a result, the DAMOS scheme can show unexpected
aggressiveness.  Do the update.

The user impact is not catastrophic.  No leak or crash happens.  Doing
the quota goal online commit that can reproduce the issue is expected to
be not common.  This issue was not found by real users but the AI
review.  That said, the issue can reliably be reproduced.

This issue was discovered [1] by Sashiko.

[1] https://lore.kkernel.org/20260827045035.94611-1-sj@kernel.org

Fixes: 9138e27a3bc3 ("mm/damon: add node_eligible_mem_bp goal metric")
Cc: <stable@vger.kernel.org> # 7.2.x
Signed-off-by: SJ Park <sj@kernel.org>
---
 mm/damon/core.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index 39605e64dabf2..b3f8bea774c2d 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -1136,6 +1136,9 @@ static void damos_commit_quota_goal_union(
 		dst->nid = src->nid;
 		dst->memcg_id = src->memcg_id;
 		break;
+	case DAMOS_QUOTA_NODE_ELIGIBLE_MEM_BP:
+		dst->nid = src->nid;
+		break;
 	default:
 		break;
 	}
-- 
2.47.3