linux-kernel - Re: [PATCH 9/12] memcg allocate all page

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Date:	Fri, 26 Sep 2008 15:54:33 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Daisuke Nishimura <nishimura@....nes.nec.co.jp>
Cc:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"balbir@...ux.vnet.ibm.com" <balbir@...ux.vnet.ibm.com>,
	"xemul@...nvz.org" <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: [PATCH 9/12] memcg allocate all page_cgroup at boot

On Fri, 26 Sep 2008 14:54:22 +0900
Daisuke Nishimura <nishimura@....nes.nec.co.jp> wrote:
> > >    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?
> 
Maybe yes. we treat swapcache under lock_page().

> If so, how about handling swapcache instead of adding new interface?
> I think it can be done independent of mem+swap.
> 
Hmm, worth to be considered. But I'll reuse the interface itself for othres
(shmem, migrate, move_account etc)
But, in previous trial of SwapCache handling, we saw many troubles.
Then, I'd like to go carefully step by step to handle that.

Thanks,
-Kame

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