Re: [PATCH 9/12] memcg allocate all page_cgroup at boot

[KAMEZAWA Hiroyuki]

On Fri, 26 Sep 2008 10:43:36 +0900
KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote:
> > > -	/*
> > > -	 * Check if our page_cgroup is valid
> > > -	 */
> > > -	lock_page_cgroup(page);
> > > -	pc = page_get_page_cgroup(page);
> > > -	if (unlikely(!pc))
> > > -		goto unlock;
> > > -
> > > -	VM_BUG_ON(pc->page != page);
> > > +	pc = lookup_page_cgroup(page);
> > > +	if (unlikely(!pc || !PageCgroupUsed(pc)))
> > > +		return;
> > > +	preempt_disable();
> > > +	lock_page_cgroup(pc);
> > > +	if (unlikely(page_mapped(page))) {
> > > +		unlock_page_cgroup(pc);
> > > +		preempt_enable();
> > > +		return;
> > > +	}
> > Just for clarification, in what sequence will the page be mapped here?
> > mem_cgroup_uncharge_page checks whether the page is mapped.
> > 
> Please think about folloing situation.
> 
>    There is a SwapCache which is referred from 2 process, A, B.
>    A maps it.
>    B doesn't maps it.
> 
>    And now, process A exits.
> 
> 	CPU0(process A)				CPU1 (process B)
>  
>     zap_pte_range()
>     => page remove from rmap			=> charge() (do_swap_page)
> 	=> set page->mapcount->0          	
> 		=> uncharge()			=> set page->mapcount=1
> 
> This race is what patch 12/12 is fixed.
> This only happens on cursed SwapCache.
> 
Sorry, my brain seems to be sleeping.. above page_mapped() check doesn't
help this situation. Maybe this page_mapped() check is not necessary
because it's of no use.

I think this kind of problem will not be fixed until we handle SwapCache.


Thanks,
-Kame












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