Re: [PATCH] kmemcg: don't allocate extra memory for root memcg_cache_params

[Andrey Wagin <avagin@xxxxxxxxx>]

2013/8/15 Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>

On Wed, 14 Aug 2013 14:31:21 +0400 Andrey Vagin <avagin@xxxxxxxxxx> wrote:

> The memcg_cache_params structure contains the common part and the union,
> which represents two different types of data: one for root cashes and
> another for child caches.
>
> The size of child data is fixed. The size of the memcg_caches array is
> calculated in runtime.
>
> Currently the size of memcg_cache_params for root caches is calculated
> incorrectly, because it includes the size of parameters for child caches.
>
> ssize_t size = memcg_caches_array_size(num_groups);
> size *= sizeof(void *);
>
> size += sizeof(struct memcg_cache_params);
>

> ...

>
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -3140,7 +3140,7 @@ int memcg_update_cache_size(struct kmem_cache *s, int num_groups)
>               ssize_t size = memcg_caches_array_size(num_groups);
>
>               size *= sizeof(void *);
> -             size += sizeof(struct memcg_cache_params);
> +             size += sizeof(offsetof(struct memcg_cache_params, memcg_caches));

This looks wrong. offsetof() returns size_t, so this is equivalent to

                size += sizeof(size_t);

sizeof doesn't have to be here. I will resend this patch. Thanks.

size += offsetof(struct memcg_cache_params, memcg_caches)

>               s->memcg_params = kzalloc(size, GFP_KERNEL);
>               if (!s->memcg_params) {
> @@ -3183,13 +3183,16 @@ int memcg_update_cache_size(struct kmem_cache *s, int num_groups)
>  int memcg_register_cache(struct mem_cgroup *memcg, struct kmem_cache *s,
>                        struct kmem_cache *root_cache)
>  {
> -     size_t size = sizeof(struct memcg_cache_params);
> +     size_t size;
>
>       if (!memcg_kmem_enabled())
>               return 0;
>
> -     if (!memcg)
> +     if (!memcg) {
> +             size = offsetof(struct memcg_cache_params, memcg_caches);
>               size += memcg_limited_groups_array_size * sizeof(void *);
> +     } else
> +             size = sizeof(struct memcg_cache_params);
>
>       s->memcg_params = kzalloc(size, GFP_KERNEL);
>       if (!s->memcg_params)