The rcu_read_lock/unlock only can guarantee that the memcg will not be
freed, but it cannot guarantee the success of css_get (which is in the
refill_stock when cached memcg changed) to memcg.
rcu_read_lock()
memcg = obj_cgroup_memcg(old)
__memcg_kmem_uncharge(memcg)
refill_stock(memcg)
if (stock->cached != memcg)
// css_get can change the ref counter from 0 back to 1.
css_get(&memcg->css)
rcu_read_unlock()
This fix is very like the commit:
eefbfa7fd678 ("mm: memcg/slab: fix use after free in obj_cgroup_charge")
Fix this by holding a reference to the memcg which is passed to the
__memcg_kmem_uncharge() before calling __memcg_kmem_uncharge().
Fixes: 3de7d4f25a74 ("mm: memcg/slab: optimize objcg stock draining")
Signed-off-by: Muchun Song <[email protected]>
---
mm/memcontrol.c | 10 +++++++++-
1 file changed, 9 insertions(+), 1 deletion(-)
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 845eec01ef9d..2cda76ff0629 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3181,9 +3181,17 @@ static void drain_obj_stock(struct memcg_stock_pcp
*stock)
unsigned int nr_bytes = stock->nr_bytes & (PAGE_SIZE - 1);
if (nr_pages) {
+ struct mem_cgroup *memcg;
+
rcu_read_lock();
- __memcg_kmem_uncharge(obj_cgroup_memcg(old), nr_pages);
+retry:
+ memcg = obj_cgroup_memcg(old);
+ if (unlikely(!css_tryget(&memcg->css)))
+ goto retry;
rcu_read_unlock();
+
+ __memcg_kmem_uncharge(memcg, nr_pages);
+ css_put(&memcg->css);
}
/*
--
2.11.0
