The commit b74a120bcf507 ("mm/damon/core: implement
DAMOS_QUOTA_NODE_MEMCG_USED_BP") added accesses to memcg structure
without getting reference to it. This is unsafe. Let's get the reference
before accessing the memcg.

Fixes: b74a120bcf507 ("mm/damon/core: implement DAMOS_QUOTA_NODE_MEMCG_USED_BP")
Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
Reviewed-by: SeongJae Park <sj@kernel.org>
---
Changes since v1:
- Changed the subject as requested by SJ.

 mm/damon/core.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index 4ad5f290d382..89982e0229f0 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -2051,13 +2051,15 @@ static unsigned long damos_get_node_memcg_used_bp(
 
 	rcu_read_lock();
 	memcg = mem_cgroup_from_id(goal->memcg_id);
-	rcu_read_unlock();
-	if (!memcg) {
+	if (!memcg || !mem_cgroup_tryget(memcg)) {
+		rcu_read_unlock();
 		if (goal->metric == DAMOS_QUOTA_NODE_MEMCG_USED_BP)
 			return 0;
 		else	/* DAMOS_QUOTA_NODE_MEMCG_FREE_BP */
 			return 10000;
 	}
+	rcu_read_unlock();
+
 	mem_cgroup_flush_stats(memcg);
 	lruvec = mem_cgroup_lruvec(memcg, NODE_DATA(goal->nid));
 	used_pages = lruvec_page_state(lruvec, NR_ACTIVE_ANON);
@@ -2065,6 +2067,8 @@ static unsigned long damos_get_node_memcg_used_bp(
 	used_pages += lruvec_page_state(lruvec, NR_ACTIVE_FILE);
 	used_pages += lruvec_page_state(lruvec, NR_INACTIVE_FILE);
 
+	mem_cgroup_put(memcg);
+
 	si_meminfo_node(&i, goal->nid);
 	if (goal->metric == DAMOS_QUOTA_NODE_MEMCG_USED_BP)
 		numerator = used_pages;
-- 
2.47.3

On Wed, 24 Dec 2025 16:29:04 -0800 Shakeel Butt <shakeel.butt@linux.dev> wrote:

> The commit b74a120bcf507 ("mm/damon/core: implement
> DAMOS_QUOTA_NODE_MEMCG_USED_BP") added accesses to memcg structure
> without getting reference to it. This is unsafe. Let's get the reference
> before accessing the memcg.
> 
> Fixes: b74a120bcf507 ("mm/damon/core: implement DAMOS_QUOTA_NODE_MEMCG_USED_BP")
> Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
> Reviewed-by: SeongJae Park <sj@kernel.org>
> ---
> Changes since v1:
> - Changed the subject as requested by SJ.

Thank you, Shakeel!


Thanks,
SJ

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