[Devel] [PATCH vz7] mm/slab: use monotonic serial number for naming per memcg caches

[Konstantin Khorenko]

Currently, we use mem_cgroup->kmemcg_id to guarantee kmem_cache->name
uniqueness. Unfortunately kmemcg_id release happens on cgroup offline
when kmem caches are  still alive and their sysfs entries are not removed.

Thus it's possible to start/suspend/resume/stop a Container and get into
situation when mem cgroup is offline (thus mem_cgroup::kmemcg_id is
released), but not freed and corresponding slab caches are alive along
with their sysfs entries like:
  /sys/kernel/slab/filp/cgroup/filp($KMEMCG_ID:$MEMCG_NAME)

If we start a Container after that, new memory cgroup is created and
gets same kmemcg_id, new kmem_cache is created and sysfs_slab_add()
fails due to duplicate name provided which triggers a warning:

  WARNING: CPU: 0 PID: 31346 at lib/kobject.c:239
    kobject_add_internal+0x6ef/0x910
  kobject_add_internal failed for kmalloc-512(3:100) with -EEXIST, don't
    try to register things with the same name in the same directory.

In older kernels we use css_id(mem_cgroup_css(memcg)), but now cssid is
dropped.
In mainstream monotonic counter css->serial_nr is used instead,
so let's add monotonic serial_nr as well, but as nobody needs it at the
moment except memcg, let it be part of struct mem_cgroup.

https://jira.sw.ru/browse/PSBM-89047

Signed-off-by: Konstantin Khorenko <khorenko at virtuozzo.com>
---
 include/linux/memcontrol.h | 11 +++++++++++
 mm/memcontrol.c            | 18 ++++++++++++++++++
 mm/slab_common.c           | 12 ++++++++++--
 3 files changed, 39 insertions(+), 2 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 640a5802e398..56eb202ac654 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -549,6 +549,8 @@ bool __memcg_kmem_newpage_charge(struct page *page, gfp_t gfp, int order);
 void __memcg_kmem_uncharge_pages(struct page *page, int order);
 
 int memcg_cache_id(struct mem_cgroup *memcg);
+void memcg_set_serial_nr(struct mem_cgroup *memcg, u64 val);
+u64 memcg_get_serial_nr(struct mem_cgroup *memcg);
 
 struct kmem_cache *
 __memcg_kmem_get_cache(struct kmem_cache *cachep, gfp_t gfp);
@@ -685,6 +687,15 @@ static inline int memcg_cache_id(struct mem_cgroup *memcg)
 	return -1;
 }
 
+static inline void memcg_set_serial_nr(struct mem_cgroup *memcg, u64 val)
+{
+}
+
+static inline u64 memcg_get_serial_nr(struct mem_cgroup *memcg)
+{
+	return 0;
+}
+
 static inline void memcg_get_cache_ids(void)
 {
 }
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index e57263fea566..2061d5b116bf 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -404,6 +404,14 @@ struct mem_cgroup {
 #if defined(CONFIG_MEMCG_KMEM)
         /* Index in the kmem_cache->memcg_params.memcg_caches array */
 	int kmemcg_id;
+	/*
+	 * Monotonically increasing unique serial number which defines a
+	 * uniform order among all mem cgroups.
+	 * It's used to construct unique kmem_cache name which should remain
+	 * unique after mem cgroup offline when kmemcg_id is released.
+	 */
+	u64 serial_nr;
+
 	/* List of memcgs sharing the same kmemcg_id */
 	struct list_head kmemcg_sharers;
 #endif
@@ -3432,6 +3440,16 @@ int memcg_cache_id(struct mem_cgroup *memcg)
 	return memcg ? memcg->kmemcg_id : -1;
 }
 
+void memcg_set_serial_nr(struct mem_cgroup *memcg, u64 val)
+{
+	memcg->serial_nr = val;
+}
+
+u64 memcg_get_serial_nr(struct mem_cgroup *memcg)
+{
+	return memcg->serial_nr;
+}
+
 static int memcg_alloc_cache_id(void)
 {
 	int id, size;
diff --git a/mm/slab_common.c b/mm/slab_common.c
index 1b2a7d208596..24abd2e88efe 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -421,6 +421,7 @@ static void do_kmem_cache_release(struct list_head *release,
 void memcg_create_kmem_cache(struct mem_cgroup *memcg,
 			     struct kmem_cache *root_cache)
 {
+	static atomic64_t serial_nr_cursor = ATOMIC64_INIT(0);
 	static char memcg_name_buf[NAME_MAX + 1]; /* protected by slab_mutex */
 	struct memcg_cache_array *arr;
 	struct kmem_cache *s = NULL;
@@ -453,8 +454,15 @@ void memcg_create_kmem_cache(struct mem_cgroup *memcg,
 	strlcpy(memcg_name_buf, cgroup_name(mem_cgroup_css(memcg)->cgroup),
 		NAME_MAX + 1);
 	rcu_read_unlock();
-	cache_name = kasprintf(GFP_KERNEL, "%s(%d:%s)", root_cache->name,
-			       idx, memcg_name_buf);
+
+	/*
+	 * Assign a monotonically increasing serial number.
+	 * It guarantees kmem_cache name uniqueness even if a mem cgroup goes
+	 * offline, releases kmemcg_id and new mem cgroup gets same kmemcg_id.
+	 */
+	memcg_set_serial_nr(memcg, atomic64_inc_return(&serial_nr_cursor));
+	cache_name = kasprintf(GFP_KERNEL, "%s(%llu:%s)", root_cache->name,
+			       memcg_get_serial_nr(memcg), memcg_name_buf);
 	if (!cache_name)
 		goto out_unlock;
 
-- 
2.15.1