[PATCH] mm/memcg: fix NULL nodeinfo[] dereference on late-onlined nodes

memcg->nodeinfo[] entries are allocated only for nodes present at
css_alloc time. When a node is onlined after a memcg is created its
nodeinfo[] slot remains NULL. Two call sites dereference these slots
unconditionally:

lruvec_stat_mod_folio() calls mem_cgroup_lruvec() which reads
memcg->nodeinfo[pgdat->node_id] without a NULL check. On a system
where a node is onlined after the memcg is created, any folio stat
update for that node crashes with a NULL pointer dereference:

  Unable to handle kernel paging request at virtual address ffffffbebf7e1908
  pc : lruvec_stat_mod_folio+0xf0/0x444   [6.18.21-android17-5]
  lr : lruvec_stat_mod_folio+0x88/0x444
  Call trace:
   lruvec_stat_mod_folio+0xf0/0x444
   folio_add_new_anon_rmap+0xac/0x2b8
   do_wp_page+0x768/0xc80
   handle_mm_fault+0x37c/0x8c4
   do_page_fault+0x140/0xa1c
   do_mem_abort+0x54/0x74
   el0_da+0x48/0x8c
   el0t_64_sync_handler+0x20/0x130
   el0t_64_sync+0x1c4/0x1c8

__invalidate_reclaim_iterators() iterates for_each_node() and reads
from->nodeinfo[nid]->iter without checking for NULL. for_each_node()
visits all possible nodes, so this is reachable whenever a node is
onlined after the memcg was created.

Fix lruvec_stat_mod_folio() by checking nodeinfo[pgdat->node_id]
directly and falling back to mod_node_page_state() when NULL, mirroring
the existing !memcg early-return path. The fallback must not go through
mod_lruvec_state() since that calls mod_memcg_lruvec_state() which uses
container_of() to recover the mem_cgroup_per_node from the lruvec
pointer; passing &pgdat->__lruvec there produces a garbage pointer.
When nodeinfo[nid] is NULL the memcg has no per-node accounting
structure for that node, so node-level accounting is correct.

Fix __invalidate_reclaim_iterators() by skipping NULL nodeinfo[] slots.

Also switch lruvec_stat_mod_folio() to folio_memcg_check() which uses
READ_ONCE() to safely read folio->memcg_data in an unlocked context.

Fixes: 6c77b607ee26 ("mm: kill lock|unlock_page_memcg()")
Cc: [email protected]
Assisted-by: pi:claude-sonnet-4-5
Signed-off-by: Prakash Gupta <[email protected]>
---
 mm/memcontrol.c | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 6dc4888a90f3..2a6f02956b89 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -977,11 +977,12 @@ void lruvec_stat_mod_folio(struct folio *folio, enum node_stat_item idx,
 			     int val)
 {
 	struct mem_cgroup *memcg;
+	struct mem_cgroup_per_node *mz;
 	pg_data_t *pgdat = folio_pgdat(folio);
 	struct lruvec *lruvec;
 
 	rcu_read_lock();
-	memcg = folio_memcg(folio);
+	memcg = folio_memcg_check(folio);
 	/* Untracked pages have no memcg, no lruvec. Update only the node */
 	if (!memcg) {
 		rcu_read_unlock();
@@ -989,7 +990,18 @@ void lruvec_stat_mod_folio(struct folio *folio, enum node_stat_item idx,
 		return;
 	}
 
-	lruvec = mem_cgroup_lruvec(memcg, pgdat);
+	mz = memcg->nodeinfo[pgdat->node_id];
+	if (unlikely(!mz)) {
+		/*
+		 * nodeinfo[nid] is NULL when a node is onlined after the
+		 * memcg was created. Fall back to node-level accounting.
+		 */
+		rcu_read_unlock();
+		mod_node_page_state(pgdat, idx, val);
+		return;
+	}
+
+	lruvec = &mz->lruvec;
 	mod_lruvec_state(lruvec, idx, val);
 	rcu_read_unlock();
 }
@@ -1320,6 +1332,8 @@ static void __invalidate_reclaim_iterators(struct mem_cgroup *from,
 
 	for_each_node(nid) {
 		mz = from->nodeinfo[nid];
+		if (!mz)
+			continue;
 		iter = &mz->iter;
 		cmpxchg(&iter->position, dead_memcg, NULL);
 	}

---
base-commit: 075b74841bd0065a3bda3440873c747938e69b68
change-id: 20260808-memcg-nodeinfo-null-guard-dce5f63a2349

Best regards,
-- 
Prakash Gupta <[email protected]>