2013/8/13 Andrew Morton [off-list ref]:

On Fri,  9 Aug 2013 00:51:26 +0400 Andrey Vagin [off-list ref] wrote:

quoted

struct memcg_cache_params {
        bool is_root_cache;
        union {
                struct kmem_cache *memcg_caches[0];
                struct {
                        struct mem_cgroup *memcg;
                        struct list_head list;
                        struct kmem_cache *root_cache;
                        bool dead;
                        atomic_t nr_pages;
                        struct work_struct destroy;
                };
        };
};

This union is a bit dangerous. //Andrew Morton

The first problem was fixed in v3.10-rc5-67-gf101a94.
The second problem is that the size of memory for root
caches is calculated incorrectly:

      ssize_t size = memcg_caches_array_size(num_groups);

      size *= sizeof(void *);
      size += sizeof(struct memcg_cache_params);

The last line should be fixed like this:
      size += offsetof(struct memcg_cache_params, memcg_caches)

Andrew suggested to rework this code without union and
this patch tries to do that.

hm, did I?

I reread your messages. I have seen in it, what I want. Sorry, you
suggested to rework this code how you explained bellow in this
message. "without union" is my fantasy.
http://lkml.indiana.edu/hypermail/linux/kernel/1305.3/01985.html

quoted

This patch removes is_root_cache and union. The size of the
memcg_cache_params structure is not changed.

It's a bit sad to consume more space because we're sucky programmers.
It would be better to retain the union and to stop writing buggy code
to handle it!

I decided to implement this approach, because it doesn't increase
memory consumptions. This patch is replace is_root_cache on a pointer,
but due to alignment the size of the structure is not changed.

but the size of struct kmem_cache is increased on one pointer, if
accounting of kernel memory is not enabled.

The overhead of this patch on a real system is about 1K if the kernel
memory accounting is disabled and the overhead is zero after enabling
the accounting.

Maybe there are things we can do to reduce the likelihood of people
mishandling the union - don't use anonymous fields, name each member,
access it via helper functions, etc.

Ok, I wil try this way.

Thanks,
Andrey