Because the ultimate goal of the kmem tracking in memcg is to
track slab pages as well, we can't guarantee that we'll always
be able to point a page to a particular process, and migrate
the charges along with it - since in the common case, a page
will contain data belonging to multiple processes.

Because of that, when we destroy a memcg, we only make sure
the destruction will succeed by discounting the kmem charges
from the user charges when we try to empty the cgroup.

Signed-off-by: Glauber Costa <redacted>
CC: Christoph Lameter <redacted>
CC: Pekka Enberg <redacted>
CC: Michal Hocko <redacted>
CC: Kamezawa Hiroyuki <redacted>
CC: Johannes Weiner <hannes@cmpxchg.org>
CC: Suleiman Souhlal <redacted>
---
 mm/memcontrol.c |   10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index a6a440b..bb9b6fe 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c

@@ -598,6 +598,11 @@ static void disarm_kmem_keys(struct mem_cgroup *memcg)
 {
 	if (test_bit(KMEM_ACCOUNTED_THIS, &memcg->kmem_accounted))
 		static_key_slow_dec(&mem_cgroup_kmem_enabled_key);
+	/*
+	 * This check can't live in kmem destruction function,
+	 * since the charges will outlive the cgroup
+	 */
+	BUG_ON(res_counter_read_u64(&memcg->kmem, RES_USAGE) != 0);