Re: [PATCH 3.6/13] memcg: add function to move account.

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Wed, 24 Sep 2008 15:50:11 +0900
Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp> wrote:

> On Mon, 22 Sep 2008 20:09:48 +0900, KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:
> > Sorry, this patchs is after "3.5" , before "4"....
> > 
> > ==
> > This patch provides a function to move account information of a page between
> > mem_cgroups.
> > 
> > This moving of page_cgroup is done under
> >  - the page is locked.
> >  - lru_lock of source/destination mem_cgroup is held.
> > 
> > Then, a routine which touches pc->mem_cgroup without page_lock() should
> > confirm pc->mem_cgroup is still valid or not. Typlical code can be following.
> > 
> > (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);
> > 
> 
> Is this check needed?
> Both move_lists and move_account takes page_cgroup lock.
> 
__mem_cgroup_move_lists() doesn't take.
But yes, if you know what it does, you can reduce checks.
Above is example.


Thanks,
-Kame


> 
> Thanks,
> Daisuke Nishimura.
> 
> > If you find page_cgroup from mem_cgroup's LRU under mz->lru_lock, you don't
> > have to worry about anything.
> > 
> > Changelog: (v3) -> (v4)
> >   - no changes.
> > Changelog: (v2) -> (v3)
> >   - 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@jp.fujitsu.com>
> > 
> >  mm/memcontrol.c |   85 ++++++++++++++++++++++++++++++++++++++++++++++++++++++--
> >  1 file changed, 82 insertions(+), 3 deletions(-)
> > 
> > Index: mmotm-2.6.27-rc6+/mm/memcontrol.c
> > ===================================================================
> > --- mmotm-2.6.27-rc6+.orig/mm/memcontrol.c
> > +++ mmotm-2.6.27-rc6+/mm/memcontrol.c
> > @@ -424,6 +424,7 @@ 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;
> >  
> > @@ -442,9 +443,14 @@ 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);
> > @@ -565,6 +571,71 @@ 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. lock the page by lock_page().
> > + * 2. disable irq.
> > + * 3. lru_lock of old mem_cgroup should be held.
> > + * 4. pc is guaranteed to be valid and on mem_cgroup's LRU.
> > + *
> > + * Because we cannot call try_to_free_page() here, the caller must guarantee
> > + * this moving of change never 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());
> > +	VM_BUG_ON(!PageLocked(page));
> > +
> > +	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;
> > +	}
> > +	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)) {
> > +		__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
> > @@ -752,16 +823,24 @@ __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() */
> > +		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);
> >  
> > 
> 

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