Re: Re: [PATCH 7/12] memcg add function to move account

[kamezawa . hiroyu]

----- Original Message -----

>KAMEZAWA Hiroyuki wrote:
>> This patch provides a function to move account information of a page betwee
n
>> mem_cgroups.
>> 
>
>What is the interface for moving the accounting? Is it an explicit call like
>force_empty?

My final purpose is moving account at attach_task().

> The other concern I have is about merging two LRU lists, when we
>move LRU pages from one mem_cgroup to another, where do we add them? 
> To the head
>or tail? I think we need to think about it and document it well.
>
Hmm, good point. considering force_empty, head is better.
(But I don't think about this seriously because I assumed root cgroup
 is unlimited)

I don't think this kind of internal behavior should not be documented as UI
while it's hot.

>The other thing is that once we have mult-hierarchy support (which we really
>need), we need to move the accounting to the parent instead of root.
>
yes. of course. please do as you like if this goes.
adding logic to do so needs more patch, so please wait.

But, IMHO, just use try_to_free_pages() is better for hierarchy, maybe.

>> This moving of page_cgroup is done under
>>  - lru_lock of source/destination mem_cgroup is held.
>
>I suppose you mean and instead of either for the lru_lock
>
I'll fix this comment.

>>  - lock_page_cgroup() is held.
>> 
>> Then, a routine which touches pc->mem_cgroup without lock_page_cgroup() sho
uld
>> confirm pc->mem_cgroup is still valid or not. Typlical code can be followin
g.
>> 
>> (while page is not under lock_page())
>> 	mem = pc->mem_cgroup;
>> 	mz = page_cgroup_zoneinfo(pc)
>> 	spin_lock_irqsave(&mz->lru_lock);
>> 	if (pc->mem_cgroup == mem)
>> 		...../* some list handling */
>> 	spin_unlock_irq(&mz->lru_lock);
>> 
>> Or better way is
>> 	lock_page_cgroup(pc);
>> 	....
>> 	unlock_page_cgroup(pc);
>> 
>> But you should confirm the nest of lock and avoid deadlock.
>> (trylock is better if it's ok.)
>> 
>> If you find page_cgroup from mem_cgroup's LRU under mz->lru_lock,
>> you don't have to worry about what pc->mem_cgroup points to.
>> 
>> Changelog: (v4) -> (v5)
>>   - check for lock_page() is removed.
>>   - rewrote description.
>> 
>> Changelog: (v2) -> (v4)
>>   - added lock_page_cgroup().
>>   - splitted out from new-force-empty patch.
>>   - added how-to-use text.
>>   - fixed race in __mem_cgroup_uncharge_common().
>> 
>> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
>> 
>>  mm/memcontrol.c |   84 +++++++++++++++++++++++++++++++++++++++++++++++++++
+++--
>>  1 file changed, 81 insertions(+), 3 deletions(-)
>> 
>> Index: mmotm-2.6.27-rc7+/mm/memcontrol.c
>> ===================================================================
>> --- mmotm-2.6.27-rc7+.orig/mm/memcontrol.c
>> +++ mmotm-2.6.27-rc7+/mm/memcontrol.c
>> @@ -426,6 +426,7 @@ int task_in_mem_cgroup(struct task_struc
>>  void mem_cgroup_move_lists(struct page *page, enum lru_list lru)
>>  {
>>  	struct page_cgroup *pc;
>> +	struct mem_cgroup *mem;
>>  	struct mem_cgroup_per_zone *mz;
>>  	unsigned long flags;
>> 
>> @@ -444,9 +445,14 @@ void mem_cgroup_move_lists(struct page *
>> 
>>  	pc = page_get_page_cgroup(page);
>>  	if (pc) {
>> +		mem = pc->mem_cgroup;
>>  		mz = page_cgroup_zoneinfo(pc);
>>  		spin_lock_irqsave(&mz->lru_lock, flags);
>> -		__mem_cgroup_move_lists(pc, lru);
>> +		/*
>> +		 * check against the race with move_account.
>> +		 */
>> +		if (likely(mem == pc->mem_cgroup))
>> +			__mem_cgroup_move_lists(pc, lru);
>>  		spin_unlock_irqrestore(&mz->lru_lock, flags);
>>  	}
>>  	unlock_page_cgroup(page);
>> @@ -567,6 +573,70 @@ unsigned long mem_cgroup_isolate_pages(u
>>  	return nr_taken;
>>  }
>> 
>> +/**
>> + * mem_cgroup_move_account - move account of the page
>> + * @page ... the target page of being moved.
>> + * @pc   ... page_cgroup of the page.
>> + * @from ... mem_cgroup which the page is moved from.
>> + * @to   ... mem_cgroup which the page is moved to.
>> + *
>> + * The caller must confirm following.
>> + * 1. disable irq.
>> + * 2. lru_lock of old mem_cgroup should be held.
>> + * 3. pc is guaranteed to be valid and on mem_cgroup's LRU.
>> + *
>> + * Because we cannot call try_to_free_page() here, the caller must guarant
ee
>> + * this moving of charge never fails. (if charge fails, this call fails.)
>> + * Currently this is called only against root cgroup.
>> + * which has no limitation of resource.
>> + * Returns 0 at success, returns 1 at failure.
>> + */
>> +int mem_cgroup_move_account(struct page *page, struct page_cgroup *pc,
>> +	struct mem_cgroup *from, struct mem_cgroup *to)
>> +{
>> +	struct mem_cgroup_per_zone *from_mz, *to_mz;
>> +	int nid, zid;
>> +	int ret = 1;
>> +
>> +	VM_BUG_ON(!irqs_disabled());
>> +
>> +	nid = page_to_nid(page);
>> +	zid = page_zonenum(page);
>> +	from_mz =  mem_cgroup_zoneinfo(from, nid, zid);
>> +	to_mz =  mem_cgroup_zoneinfo(to, nid, zid);
>> +
>> +	if (res_counter_charge(&to->res, PAGE_SIZE)) {
>> +		/* Now, we assume no_limit...no failure here. */
>> +		return ret;
>> +	}
>
>Please BUG_ON() if the charging fails, we can be sure we catch assumptions
> are broken.
>
I'll remove this charge() and change protocol to be 
"the page should be pre-charged to destination cgroup before calling move."


>> +	if (!try_lock_page_cgroup(page)) {
>> +		res_counter_uncharge(&to->res, PAGE_SIZE);
>> +		return ret;
>> +	}
>> +
>> +	if (page_get_page_cgroup(page) != pc) {
>> +		res_counter_uncharge(&to->res, PAGE_SIZE);
>> +		goto out;
>> +	}
>> +
>> +	if (spin_trylock(&to_mz->lru_lock)) {
>
>The spin_trylock is to avoid deadlocks, right?
>
yes.

>> +		__mem_cgroup_remove_list(from_mz, pc);
>> +		css_put(&from->css);
>> +		res_counter_uncharge(&from->res, PAGE_SIZE);
>> +		pc->mem_cgroup = to;
>> +		css_get(&to->css);
>> +		__mem_cgroup_add_list(to_mz, pc);
>> +		ret = 0;
>> +		spin_unlock(&to_mz->lru_lock);
>> +	} else {
>> +		res_counter_uncharge(&to->res, PAGE_SIZE);
>> +	}
>> +out:
>> +	unlock_page_cgroup(page);
>> +
>> +	return ret;
>> +}
>> +
>>  /*
>>   * Charge the memory controller for page usage.
>>   * Return
>> @@ -754,16 +824,24 @@ __mem_cgroup_uncharge_common(struct page
>>  	if ((ctype == MEM_CGROUP_CHARGE_TYPE_MAPPED)
>>  	    && ((PageCgroupCache(pc) || page_mapped(page))))
>>  		goto unlock;
>> -
>> +retry:
>> +	mem = pc->mem_cgroup;
>>  	mz = page_cgroup_zoneinfo(pc);
>>  	spin_lock_irqsave(&mz->lru_lock, flags);
>> +	if (ctype == MEM_CGROUP_CHARGE_TYPE_MAPPED &&
>> +	    unlikely(mem != pc->mem_cgroup)) {
>> +		/* MAPPED account can be done without lock_page().
>> +		   Check race with mem_cgroup_move_account() */
>
>Coding style above is broken.
will fix.

>Can this race really occur?
yes, I think so. please check SwapCache behavior.
But This check may disappear after I rewrite the whole series.

>Why do we get mem before acquiring the mz->lru_lock? We don't seem to 
>be using it.
It's costly to call page_cgroup_zoneinfo() again. so just saves mem and
compare pc->mem_cgroup.

Thanks,
-Kame

>> +		spin_unlock_irqrestore(&mz->lru_lock, flags);
>> +		goto retry;
>> +	}
>>  	__mem_cgroup_remove_list(mz, pc);
>>  	spin_unlock_irqrestore(&mz->lru_lock, flags);
>> 
>>  	page_assign_page_cgroup(page, NULL);
>>  	unlock_page_cgroup(page);
>> 
>> -	mem = pc->mem_cgroup;
>> +
>>  	res_counter_uncharge(&mem->res, PAGE_SIZE);
>>  	css_put(&mem->css);
>> 
>> 
>
>
>-- 
>	Balbir

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