memcg: allow a memcg with kmem charges to be destructed

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 <glommer@parallels.com>
Acked-by: Kamezawa Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Reviewed-by: Michal Hocko <mhocko@suse.cz>
Cc: Christoph Lameter <cl@linux.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Frederic Weisbecker <fweisbec@redhat.com>
Cc: Greg Thelen <gthelen@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: JoonSoo Kim <js1304@gmail.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Cc: Pekka Enberg <penberg@cs.helsinki.fi>
Cc: Rik van Riel <riel@redhat.com>
Cc: Suleiman Souhlal <suleiman@google.com>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 347024c011e2e9ab0882243bac82ade82883d632..540bac5aa5b54310ce455bbdbe9bbf55042d9b21 100644 (file)
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -545,6 +545,11 @@ static void disarm_kmem_keys(struct mem_cgroup *memcg)
 {
        if (memcg_kmem_is_active(memcg))
                static_key_slow_dec(&memcg_kmem_enabled_key);
+       /*
+        * This check can't live in kmem destruction function,
+        * since the charges will outlive the cgroup
+        */
+       WARN_ON(res_counter_read_u64(&memcg->kmem, RES_USAGE) != 0);
 }
 #else
 static void disarm_kmem_keys(struct mem_cgroup *memcg)
@@ -4014,6 +4019,7 @@ static int mem_cgroup_reparent_charges(struct mem_cgroup *memcg)
 {
        struct cgroup *cgrp = memcg->css.cgroup;
        int node, zid;
+       u64 usage;
 
        do {
                if (cgroup_task_count(cgrp) || !list_empty(&cgrp->children))
@@ -4036,13 +4042,20 @@ static int mem_cgroup_reparent_charges(struct mem_cgroup *memcg)
                cond_resched();
 
                /*
+                * Kernel memory may not necessarily be trackable to a specific
+                * process. So they are not migrated, and therefore we can't
+                * expect their value to drop to 0 here.
+                * Having res filled up with kmem only is enough.
+                *
                 * This is a safety check because mem_cgroup_force_empty_list
                 * could have raced with mem_cgroup_replace_page_cache callers
                 * so the lru seemed empty but the page could have been added
                 * right after the check. RES_USAGE should be safe as we always
                 * charge before adding to the LRU.
                 */
-       } while (res_counter_read_u64(&memcg->res, RES_USAGE) > 0);
+               usage = res_counter_read_u64(&memcg->res, RES_USAGE) -
+                       res_counter_read_u64(&memcg->kmem, RES_USAGE);
+       } while (usage > 0);
 
        return 0;
 }