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);
  }
  #else
  static void disarm_kmem_keys(struct mem_cgroup *memcg)

@@ -3838,6 +3843,7 @@ static int mem_cgroup_force_empty(struct mem_cgroup *memcg, bool free_all)
  	int node, zid, shrink;
  	int nr_retries = MEM_CGROUP_RECLAIM_RETRIES;
  	struct cgroup *cgrp = memcg->css.cgroup;
+	u64 usage;
  
  	css_get(&memcg->css);
  

@@ -3877,8 +3883,10 @@ move_account:
  		if (ret == -ENOMEM)
  			goto try_to_free;
  		cond_resched();
+		usage = res_counter_read_u64(&memcg->res, RES_USAGE) -
+			res_counter_read_u64(&memcg->kmem, RES_USAGE);
  	/* "ret" should also be checked to ensure all lists are empty. */
-	} while (res_counter_read_u64(&memcg->res, RES_USAGE) > 0 || ret);
+	} while (usage > 0 || ret);
  out:
  	css_put(&memcg->css);
  	return ret;