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Date: Sat, 27 Sep 2008 17:35:45 +0900 (JST)
From: kamezawa.hiroyu@...fujitsu.com
To: balbir@...ux.vnet.ibm.com
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
linux-mm@...ck.org, nishimura@....nes.nec.co.jp, xemul@...nvz.org,
Andrew Morton <akpm@...ux-foundation.org>,
LKML <linux-kernel@...r.kernel.org>,
Dave Hansen <haveblue@...ibm.com>, ryov@...inux.co.jp
Subject: Re: Re: [PATCH 7/12] memcg add function to move account
----- 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@...fujitsu.com>
>>
>> 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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