[PATCH] mm/list_lru: drain before clearing xarray entry on reparent

[Shakeel Butt <shakeel.butt@linux.dev>]

memcg_reparent_list_lrus() clears the dying memcg's xarray entry with
xas_store(&xas, NULL) before reparenting its per-node lists into the
parent. This opens a window where a concurrent list_lru_del() arriving
for the dying memcg sees xa_load() == NULL, walks to the parent in
lock_list_lru_of_memcg(), takes the parent's per-node lock, and calls
list_del_init() on an item still physically linked on the dying
memcg's list.

If another in-flight thread holds the dying memcg's per-node lock at
the same moment (another list_lru_del, or a list_lru_walk_one running
an isolate callback), both threads modify ->next/->prev pointers on the
same physical list under different locks. Adjacent items can corrupt
each other's links.

Fix it by reversing the order: reparent each per-node list and mark the
child's list lru dead and then clear the xarray entry. Any concurrent
list_lru op that finds the still-set xarray entry either takes the dying
memcg's per-node lock (synchronizing with the drain) or sees LONG_MIN
and walks to the parent, where the items now live.

Fixes: fb56fdf8b9a2 ("mm/list_lru: split the lock to per-cgroup scope")
Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
Reported-by: Chris Mason <clm@fb.com>
---
 mm/list_lru.c | 20 +++++++++-----------
 1 file changed, 9 insertions(+), 11 deletions(-)

diff --git a/mm/list_lru.c b/mm/list_lru.c
index dd29bcf8eb5f..ae55a52307db 100644
--- a/mm/list_lru.c
+++ b/mm/list_lru.c
@@ -473,26 +473,24 @@ void memcg_reparent_list_lrus(struct mem_cgroup *memcg, struct mem_cgroup *paren
 	mutex_lock(&list_lrus_mutex);
 	list_for_each_entry(lru, &memcg_list_lrus, list) {
 		struct list_lru_memcg *mlru;
-		XA_STATE(xas, &lru->xa, memcg->kmemcg_id);
 
-		/*
-		 * Lock the Xarray to ensure no on going list_lru_memcg
-		 * allocation and further allocation will see css_is_dying().
-		 */
-		xas_lock_irq(&xas);
-		mlru = xas_store(&xas, NULL);
-		xas_unlock_irq(&xas);
+		mlru = xa_load(&lru->xa, memcg->kmemcg_id);
 		if (!mlru)
 			continue;
 
 		/*
-		 * With Xarray value set to NULL, holding the lru lock below
-		 * prevents list_lru_{add,del,isolate} from touching the lru,
-		 * safe to reparent.
+		 * Reparent each per-node list and mark the child dead
+		 * (LONG_MIN) before clearing xarray entry otherwisw a
+		 * concurrent list_lru_del() may corrupt the list if it arrives
+		 * after xarray clear but before reparenting as
+		 * lock_list_lru_of_memcg will acquire parent's lock while the
+		 * item is still on child's list.
 		 */
 		for_each_node(i)
 			memcg_reparent_list_lru_one(lru, i, &mlru->node[i], parent);
 
+		xa_erase(&lru->xa, memcg->kmemcg_id);
+
 		/*
 		 * Here all list_lrus corresponding to the cgroup are guaranteed
 		 * to remain empty, we can safely free this lru, any further
-- 
2.52.0