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

[Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>]

On Fri, 26 Sep 2008 11:05:50 +0900, KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:
> On Fri, 26 Sep 2008 10:43:36 +0900
> KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> 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.
> 
I've not fully understood yet what [12/12] does, but if we handle
swapcache properly, [12/12] would become unnecessary?

If so, how about handling swapcache instead of adding new interface?
I think it can be done independent of mem+swap.


Thanks,
Daisuke Nishimura.