Re: [RFC][PATCH 2/3] memcg: reduce size of struct page_cgroup.

[Suleiman Souhlal <suleiman@google.com>]

2012/3/19 KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>:
> Now, page_cgroup->flags has only 3bits. Considering alignment of
> struct mem_cgroup, which is allocated by kmalloc(), we can encode
> pointer to mem_cgroup and flags into a word.
>
> After this patch, pc->flags is encoded as
>
>  63                           2     0
>  | pointer to memcg..........|flags|
>
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> ---
>  include/linux/page_cgroup.h |   15 ++++++++++++---
>  1 files changed, 12 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/page_cgroup.h b/include/linux/page_cgroup.h
> index 92768cb..bca5447 100644
> --- a/include/linux/page_cgroup.h
> +++ b/include/linux/page_cgroup.h
> @@ -1,6 +1,10 @@
>  #ifndef __LINUX_PAGE_CGROUP_H
>  #define __LINUX_PAGE_CGROUP_H
>
> +/*
> + * Because these flags are encoded into ->flags with a pointer,
> + * we cannot have too much flags.
> + */
>  enum {
>        /* flags for mem_cgroup */
>        PCG_LOCK,  /* Lock for pc->mem_cgroup and following bits. */
> @@ -9,6 +13,8 @@ enum {
>        __NR_PCG_FLAGS,
>  };
>
> +#define PCG_FLAGS_MASK ((1 << __NR_PCG_FLAGS) - 1)
> +
>  #ifndef __GENERATING_BOUNDS_H
>  #include <generated/bounds.h>
>
> @@ -21,10 +27,12 @@ enum {
>  * page_cgroup helps us identify information about the cgroup
>  * All page cgroups are allocated at boot or memory hotplug event,
>  * then the page cgroup for pfn always exists.
> + *
> + * flags and a pointer to memory cgroup are encoded into ->flags.
> + * Lower 3bits are used for flags and others are used for a pointer to memcg.

Would it be worth adding a BUILD_BUG_ON(__NR_PCG_FLAGS > 3) ?

-- Suleiman

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