Re: [PATCH v4 04/14] kmem accounting basic infrastructure

for the group. Once the limit is set the accounting cannot be disabled
for that group.  This means that after the patch is applied, no
behavioral changes exists for whoever is still using memcg to control
their memory usage, until memory.kmem.limit_in_bytes is set for the
first time.

We always account to both user and kernel resource_counters. This
effectively means that an independent kernel limit is in place when the
limit is set to a lower value than the user memory. A equal or higher
value means that the user limit will always hit first, meaning that kmem
is effectively unlimited.

People who want to track kernel memory but not limit it, can set this
limit to a very high number (like RESOURCE_MAX - 1page - that no one
will ever hit, or equal to the user memory)

[ v4: make kmem files part of the main array;
      do not allow limit to be set for non-empty cgroups ]

Signed-off-by: Glauber Costa <redacted>
CC: Michal Hocko <redacted>
CC: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Kamezawa Hiroyuki <redacted>
---
 mm/memcontrol.c | 123 +++++++++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 122 insertions(+), 1 deletion(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 71d259e..ba855cc 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c

@@ -332,6 +337,26 @@ struct mem_cgroup {
 #endif
 };
 
+/* internal only representation about the status of kmem accounting. */
+enum {
+	KMEM_ACCOUNTED_ACTIVE = 0, /* accounted by this cgroup itself */
+};
+
+/* first bit */
+#define KMEM_ACCOUNTED_MASK 0x1
+
+#ifdef CONFIG_MEMCG_KMEM
+static void memcg_kmem_set_active(struct mem_cgroup *memcg)
+{
+	set_bit(KMEM_ACCOUNTED_ACTIVE, &memcg->kmem_accounted);
+}
+
+static bool memcg_kmem_is_accounted(struct mem_cgroup *memcg)
+{
+	return test_bit(KMEM_ACCOUNTED_ACTIVE, &memcg->kmem_accounted);
+}
+#endif

@@ -3947,6 +3980,58 @@ static ssize_t mem_cgroup_read(struct cgroup *cont, struct cftype *cft,
 	len = scnprintf(str, sizeof(str), "%llu\n", (unsigned long long)val);
 	return simple_read_from_buffer(buf, nbytes, ppos, str, len);
 }
+
+static int memcg_update_kmem_limit(struct cgroup *cont, u64 val)
+{
+	int ret = -EINVAL;
+#ifdef CONFIG_MEMCG_KMEM
+	struct mem_cgroup *memcg = mem_cgroup_from_cont(cont);
+	/*
+	 * For simplicity, we won't allow this to be disabled.  It also can't
+	 * be changed if the cgroup has children already, or if tasks had
+	 * already joined.
+	 *
+	 * If tasks join before we set the limit, a person looking at
+	 * kmem.usage_in_bytes will have no way to determine when it took
+	 * place, which makes the value quite meaningless.
+	 *
+	 * After it first became limited, changes in the value of the limit are
+	 * of course permitted.
+	 *
+	 * Taking the cgroup_lock is really offensive, but it is so far the only
+	 * way to guarantee that no children will appear. There are plenty of
+	 * other offenders, and they should all go away. Fine grained locking
+	 * is probably the way to go here. When we are fully hierarchical, we
+	 * can also get rid of the use_hierarchy check.
+	 */
+	cgroup_lock();
+	mutex_lock(&set_limit_mutex);
+	if (!memcg->kmem_accounted && val != RESOURCE_MAX) {