Date: Fri, 27 Jun 2008 18:29:52 +0900
From: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
To: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: "MinChan Kim" <minchan.kim@gmail.com>,
       "KOSAKI Motohiro" <kosaki.motohiro@jp.fujitsu.com>,
       LKML <linux-kernel@vger.kernel.org>, linux-mm <linux-mm@kvack.org>,
       "Andrew Morton" <akpm@linux-foundation.org>,
       "Lee Schermerhorn" <Lee.Schermerhorn@hp.com>,
       "Rik van Riel" <riel@redhat.com>
Subject: Re: [-mm][PATCH 8/10] fix shmem page migration incorrectness on
 memcgroup
Message-Id: <20080627182952.f8d2b0c3.nishimura@mxp.nes.nec.co.jp>
In-Reply-To: <20080627175201.cbe86a06.kamezawa.hiroyu@jp.fujitsu.com>
References: <20080625190750.D864.KOSAKI.MOTOHIRO@jp.fujitsu.com>
	<28c262360806262208i6791d67at446f7323ded16206@mail.gmail.com>
	<20080627142950.7A83.KOSAKI.MOTOHIRO@jp.fujitsu.com>
	<28c262360806270057w2b2d3e56ob4dde9aacf42327b@mail.gmail.com>
	<20080627175201.cbe86a06.kamezawa.hiroyu@jp.fujitsu.com>
Mime-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 2567
Lines: 73

On Fri, 27 Jun 2008 17:52:01 +0900, KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:
> On Fri, 27 Jun 2008 16:57:56 +0900
> "MinChan Kim" <minchan.kim@gmail.com> wrote:
> 
> > On Fri, Jun 27, 2008 at 2:41 PM, KOSAKI Motohiro
> > <kosaki.motohiro@jp.fujitsu.com> wrote:
> > >> > mem_cgroup_uncharge() against old page is done after radix-tree-replacement.
> > >> > And there were special handling to ingore swap-cache page. But, shmem can
> > >> > be swap-cache and file-cache at the same time. Chekcing PageSwapCache() is
> > >> > not correct here. Check PageAnon() instead.
> > >>
> > >> When/How shmem can be both swap-cache and file-cache ?
> > >> I can't understand that situation.
> > >
> > > Hi
> > >
> > > see,
> > >
> > > shmem_writepage()
> > >   -> add_to_swap_cache()
> > >      -> SetPageSwapCache()
> > >
> > >
> > > BTW: his file-cache mean !Anon, not mean !SwapBacked.
> > 
> > Hi KOSAKI-san.
> > Thanks for explaining.
> > 
> > In the migrate_page_move_mapping, the page was already locked in unmap_and_move.
> > Also, we have a lock for that page for calling shmem_writepage.
> > 
> > So I think race problem between shmem_writepage and
> > migrate_page_move_mapping don't occur.
> > But I am not sure I am right.
> > 
> > If I am wrong, could you tell me when race problem happen ? :)
> > 
> You are right. I misundestood the swap/shmem code. there is no race.
> Hmm...
> 
> But situation is a bit complicated.
> - shmem's page is charged as file-cache.
> - shmem's swap cache is still charged by mem_cgroup_cache_charge() because
>   it's implicitly (to memcg) converted to swap cache. 
> - anon's swap cache is charged by mem_cgroup_uncharge_cache_page()
> 
I'm sorry if I misunderstand something.

I think anon's swap cache is:

- charged by nowhere as "cache".
  If anon pages are also on swap cache, charges for them remain charged
  even when mem_cgroup_uncharge_page() is called, because it checks PG_swapcache.
  So, as a result, anon's swap cache is charged.
- uncharged by memcgroup_uncharge_page() in __delete_from_swap_cache()
  after clearing PG_swapcache.

right?

> So, uncharging swap-cache of shmem by mem_cgroup_uncharge_cache_page() is valid.
> Checking PageSwapCache() was bad and Cheking PageAnon() is good.
> (From maintainance view)
> 
agree.


Thanks,
Daisuke Nishimura.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/