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

[KAMEZAWA Hiroyuki <kamezawa.hiroyu-+CUm20s59erQFUHtdCDX3A@public.gmane.org>]

(2012/03/20 7:20), Suleiman Souhlal wrote:

> 2012/3/19 KAMEZAWA Hiroyuki <kamezawa.hiroyu-+CUm20s59erQFUHtdCDX3A@public.gmane.org>:
>> 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-+CUm20s59erQFUHtdCDX3A@public.gmane.org>
>> ---
>>  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) ?
> 


Ok, I'll add that.

Thanks,
-Kame