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

[Konstantin Khorenko]

On 10/15/2018 11:58 AM, Kirill Tkhai wrote:
> On 12.10.2018 18:54, Konstantin Khorenko wrote:
>> 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.
>
> Looks good. Please, look at small nits below.
>
>> 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;
>> +}
>
> Do we really need this struct mem_cgroup::serial_nr and the get/set primitives?
> Are we going to use them for something else in the future?

Not really.
i've added those get/set primitives because otherwise gcc complained about incomplete type "struct mem_cgroup",
at the moment mem_cgroup is defined in mm/memcontrol.c and i didn't want to move it to header,
so get/set wrappers provide a smaller patch.

>
>> +
>>  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));
>
> Here we are already held slab_mutex, so it looks like atomic is overkill.
> Simple u64 should be enough.

You are definitely right, thank you.

>
>> +	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;
>
> Kirill
> .
>