[v3] mm: Add RCU read brackets into memcg_uncharge_kmem()

Submitted by Kirill Tkhai on May 7, 2020, 9:49 a.m.

Details

Message ID 158884494513.26154.16840534816651860041.stgit@localhost.localdomain
State New
Series "mm: Move memcg_uncharge_kmem() to bottom of __memcg_uncharge_slab()"
Headers show

Commit Message

Kirill Tkhai May 7, 2020, 9:49 a.m.
Potentially, here can be last css refcnt, so we should
have stable memory to access parent pointer.

Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
---
 mm/memcontrol.c |    7 +++++++
 1 file changed, 7 insertions(+)

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diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 797cb8e6df6d..e82c59f6a984 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3525,7 +3525,13 @@  void memcg_uncharge_kmem(struct mem_cgroup *memcg,
 	 *
 	 * The memory barrier imposed by test&clear is paired with the
 	 * explicit one in memcg_kmem_mark_dead().
+	 *
+	 * We take RCU to remain memcg memory stable for parent dereferencing.
+	 * Parent's memory is stable even in case of page_counter_uncharge()
+	 * has decremented its last charged memory, because child dentry owns
+	 * parents's dentry refcnt.
 	 */
+	rcu_read_lock();
 	if (memcg_kmem_test_and_clear_dead(memcg))
 		css_put(&memcg->css);
 
@@ -3541,6 +3547,7 @@  void memcg_uncharge_kmem(struct mem_cgroup *memcg,
 		else
 			break;
 	}
+	rcu_read_unlock();
 }
 
 int __memcg_charge_slab(struct kmem_cache *s, gfp_t gfp, unsigned int nr_pages)