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Date: Wed, 18 Apr 2012 16:34:28 +0900
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To: Ying Han <yinghan@...gle.com>
CC: "linux-mm@...ck.org" <linux-mm@...ck.org>,
Linux Kernel <linux-kernel@...r.kernel.org>,
Michal Hocko <mhocko@...e.cz>,
Johannes Weiner <hannes@...xchg.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Glauber Costa <glommer@...allels.com>,
Konstantin Khlebnikov <khlebnikov@...nvz.org>,
Suleiman Souhlal <suleiman@...gle.com>
Subject: Re: [RFC][PATCH 3/6] memcg: add PageCgroupReset()
(2012/04/18 6:25), Ying Han wrote:
> On Wed, Mar 28, 2012 at 3:57 AM, KAMEZAWA Hiroyuki
> <kamezawa.hiroyu@...fujitsu.com> wrote:
>>
>> A commit "memcg: simplify LRU handling by new rule" removes PCG_ACCT_LRU.
>> and the bug introduced by it was fixed by "memcg: fix GPF when cgroup removal
>> races with last exit"
>>
>> This was for reducing flags on pc->flags....Now, we have 3bits of flags.
>> but this patch adds a new flag, I'm sorry. (Considering alignment of
>> kmalloc(), we'll able to have 5 bits..)
>>
>> This patch adds PCG_RESET which is similar to PCG_ACCT_LRU.
>
>
>
> This is set
>> when mem_cgroup_add_lru_list() finds we cannot trust the pc's mem_cgroup.
>
> Do we still need the new flag? I assume some of the upcoming patches
> will provide the guarantee of pc->mem_cgroup.
>
If per-lruvec locking can do reference-count GC, memcg will never be freed
there are pages which points to the memcg, and allows us to remove
whole this 'move to root' logic, I agree. We'll not require a new flag.
It means that memcg cannot be freed until there are not page(_cgroup) which
points to dead memcg. mem_cgroup_reset_owner() is removed, now.
We need to handle 2 cases.
- newly allocated pages which linked to memcg before use.
- unused pages but added to lru by some lazy logic.
And make a guarantee
- pages added to LRU of dead memcg will be freed or moved to other cgroup, soon.
I have no good idea.
Thanks,
-Kame
> --Ying
>>
>> The reason why this patch adds a (renamed) flag again is for merging
>> pc->flags and pc->mem_cgroup. Assume pc's mem_cgroup is encoded as
>>
>> mem_cgroup = pc->flags & ~0x7
>>
>> Updating multiple bits of pc->flags without talking lock_page_cgroup()
>> is very dangerous. And mem_cgroup_add_lru_list() updates pc->mem_cgroup
>> without taking lock. Then I add RESET bit. After this, pc_to_mem_cgroup()
>> is written as
>>
>> if (PageCgroupReset(pc))
>> return root_mem_cgroup;
>> return pc->mem_cgroup;
>>
>> This update of Reset bit can be done in atomic by set_bit(). And
>> cleared when USED bit is set.
>>
>> Considering kmalloc()'s alignment, having 4bits of flags will be ok....
>>
>> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
>> ---
>> include/linux/page_cgroup.h | 15 ++++++++-------
>> mm/memcontrol.c | 5 +++--
>> 2 files changed, 11 insertions(+), 9 deletions(-)
>>
>> diff --git a/include/linux/page_cgroup.h b/include/linux/page_cgroup.h
>> index 2707809..3f3b4ff 100644
>> --- a/include/linux/page_cgroup.h
>> +++ b/include/linux/page_cgroup.h
>> @@ -8,6 +8,7 @@ enum {
>> PCG_LOCK, /* Lock for pc->mem_cgroup and following bits. */
>> PCG_USED, /* this object is in use. */
>> PCG_MIGRATION, /* under page migration */
>> + PCG_RESET, /* have been reset to root_mem_cgroup */
>> __NR_PCG_FLAGS,
>> };
>>
>> @@ -70,6 +71,9 @@ SETPCGFLAG(Migration, MIGRATION)
>> CLEARPCGFLAG(Migration, MIGRATION)
>> TESTPCGFLAG(Migration, MIGRATION)
>>
>> +TESTPCGFLAG(Reset, RESET)
>> +SETPCGFLAG(Reset, RESET)
>> +
>> static inline void lock_page_cgroup(struct page_cgroup *pc)
>> {
>> /*
>> @@ -84,16 +88,13 @@ static inline void unlock_page_cgroup(struct page_cgroup *pc)
>> bit_spin_unlock(PCG_LOCK, &pc->flags);
>> }
>>
>> +extern struct mem_cgroup* root_mem_cgroup;
>>
>> static inline struct mem_cgroup* pc_to_mem_cgroup(struct page_cgroup *pc)
>> {
>> - return pc->mem_cgroup;
>> -}
>> -
>> -static inline void
>> -pc_set_mem_cgroup(struct page_cgroup *pc, struct mem_cgroup *memcg)
>> -{
>> - pc->mem_cgroup = memcg;
>> + if (likely(!PageCgroupReset(pc)))
>> + return pc->mem_cgroup;
>> + return root_mem_cgroup;
>> }
>>
>> static inline void
>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>> index d366b60..622fd2e 100644
>> --- a/mm/memcontrol.c
>> +++ b/mm/memcontrol.c
>> @@ -1080,7 +1080,8 @@ struct lruvec *mem_cgroup_lru_add_list(struct zone *zone, struct page *page,
>> * of pc's mem_cgroup safe.
>> */
>> if (!PageCgroupUsed(pc) && memcg != root_mem_cgroup) {
>> - pc_set_mem_cgroup(pc, root_mem_cgroup);
>> + /* this reset bit is cleared when the page is charged */
>> + SetPageCgroupReset(pc);
>> memcg = root_mem_cgroup;
>> }
>>
>> @@ -2626,7 +2627,7 @@ static int mem_cgroup_move_account(struct page *page,
>> __mem_cgroup_cancel_charge(from, nr_pages);
>>
>> /* caller should have done css_get */
>> - pc_set_mem_cgroup(pc, to);
>> + pc_set_mem_cgroup_and_flags(pc, to, BIT(PCG_USED) | BIT(PCG_LOCK));
>> mem_cgroup_charge_statistics(to, anon, nr_pages);
>> /*
>> * We charges against "to" which may not have any tasks. Then, "to"
>> --
>> 1.7.4.1
>>
>>
>
>
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