Re: [PATCH][for -mm] Fix and Enhancements for memory cgroup [3/6] add helper function for page_cgroup

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

On Tue, 09 Oct 2007 16:39:48 +0530
Balbir Singh <balbir@linux.vnet.ibm.com> wrote:
> > +static inline int
> > +page_cgroup_assign_new_page_cgroup(struct page *page, struct page_cgroup *pc)
> > +{
> > +	int ret = 0;
> > +
> > +	lock_page_cgroup(page);
> > +	if (!page_get_page_cgroup(page))
> > +		page_assign_page_cgroup(page, pc);
> > +	else
> > +		ret = 1;
> > +	unlock_page_cgroup(page);
> > +	return ret;
> > +}
> > +
> 
> Some comment on when the assignment can fail, for example if page
> already has a page_cgroup associated with it, would be nice.
> 
Sure ,will add.

> > +
> > +static inline struct page_cgroup *
> > +clear_page_cgroup(struct page *page, struct page_cgroup *pc)
> > +{
> > +	struct page_cgroup *ret;
> > +	/* lock and clear */
> > +	lock_page_cgroup(page);
> > +	ret = page_get_page_cgroup(page);
> > +	if (likely(ret == pc))
> > +		page_assign_page_cgroup(page, NULL);
> > +	unlock_page_cgroup(page);
> > +	return ret;
> > +}
> > +
> 
> We could add a comment stating that clearing would fail if the page's
> cgroup is not pc
> 
will add, too.

> > +
> >  static void __mem_cgroup_move_lists(struct page_cgroup *pc, bool active)
> >  {
> >  	if (active)
> > @@ -260,7 +289,7 @@ int mem_cgroup_charge(struct page *page,
> >  				gfp_t gfp_mask)
> >  {
> >  	struct mem_cgroup *mem;
> > -	struct page_cgroup *pc, *race_pc;
> > +	struct page_cgroup *pc;
> >  	unsigned long flags;
> >  	unsigned long nr_retries = MEM_CGROUP_RECLAIM_RETRIES;
> > 
> > @@ -353,24 +382,16 @@ noreclaim:
> >  		goto free_pc;
> >  	}
> > 
> > -	lock_page_cgroup(page);
> > -	/*
> > -	 * Check if somebody else beat us to allocating the page_cgroup
> > -	 */
> > -	race_pc = page_get_page_cgroup(page);
> > -	if (race_pc) {
> > -		kfree(pc);
> > -		pc = race_pc;
> > -		atomic_inc(&pc->ref_cnt);
> > -		res_counter_uncharge(&mem->res, PAGE_SIZE);
> > -		css_put(&mem->css);
> > -		goto done;
> > -	}
> > -
> >  	atomic_set(&pc->ref_cnt, 1);
> >  	pc->mem_cgroup = mem;
> >  	pc->page = page;
> > -	page_assign_page_cgroup(page, pc);
> > +	if (page_cgroup_assign_new_page_cgroup(page, pc)) {
> > +		/* race ... undo and retry */
> > +		res_counter_uncharge(&mem->res, PAGE_SIZE);
> > +		css_put(&mem->css);
> > +		kfree(pc);
> > +		goto retry;
> 
> This part is a bit confusing, why do we want to retry. If someone
> else charged the page already, we just continue, we let the other
> task take the charge and add this page to it's cgroup
> 
Okay. will add precise text.



> > +	}
> > 
> >  	spin_lock_irqsave(&mem->lru_lock, flags);
> >  	list_add(&pc->lru, &mem->active_list);
> > @@ -421,17 +442,18 @@ void mem_cgroup_uncharge(struct page_cgr
> > 
> >  	if (atomic_dec_and_test(&pc->ref_cnt)) {
> >  		page = pc->page;
> > -		lock_page_cgroup(page);
> > -		mem = pc->mem_cgroup;
> > -		css_put(&mem->css);
> > -		page_assign_page_cgroup(page, NULL);
> > -		unlock_page_cgroup(page);
> > -		res_counter_uncharge(&mem->res, PAGE_SIZE);
> > -
> > - 		spin_lock_irqsave(&mem->lru_lock, flags);
> > - 		list_del_init(&pc->lru);
> > - 		spin_unlock_irqrestore(&mem->lru_lock, flags);
> > -		kfree(pc);
> > +		/*
> > +		 * Obetaion page->cgroup and clear it under lock.
>                    ^^^^^^^^
>                    Not sure if I've come across this word before
Sorry (>_<; 
Get page->cgroup and clear it under lock.

> 
> > +		 */
> > +		if (clear_page_cgroup(page, pc) == pc) {
> 
> OK.. so we've come so far and seen that pc has changed underneath us,
> what do we do with this pc?
> 
Hmm... How about this ?
==
 if (clear_page_cgroup(page, pc) == pc) {
	/* do usual work */
 } else {
	BUG();
 }
== or BUG_ON(clear_page_cgroup(page, pc) != pc)

I have no clear idea when this race will occur.
But this "lock and clear" behavior is sane, I think.

Thanks,
-kame

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