linux-kernel - Re: [-mm][PATCH 8/10] fix shmem page migration incorrectness on memcgroup

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Date:	Fri, 27 Jun 2008 19:13:06 +0900
From:	"MinChan Kim" <minchan.kim@...il.com>
To:	"KAMEZAWA Hiroyuki" <kamezawa.hiroyu@...fujitsu.com>
Cc:	"KOSAKI Motohiro" <kosaki.motohiro@...fujitsu.com>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	"Lee Schermerhorn" <Lee.Schermerhorn@...com>,
	"Rik van Riel" <riel@...hat.com>,
	"Daisuke Nishimura" <nishimura@....nes.nec.co.jp>
Subject: Re: [-mm][PATCH 8/10] fix shmem page migration incorrectness on memcgroup

On Fri, Jun 27, 2008 at 5:52 PM, KAMEZAWA Hiroyuki
<kamezawa.hiroyu@...fujitsu.com> wrote:
> On Fri, 27 Jun 2008 16:57:56 +0900
> "MinChan Kim" <minchan.kim@...il.com> wrote:
>
>> On Fri, Jun 27, 2008 at 2:41 PM, KOSAKI Motohiro
>> <kosaki.motohiro@...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()
>
> 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)

I agree.
I also thought your patch is no problem.
It is just description problem.


> I think the patch is valid but my patch description contains wrong information.
> Andrew, could you drop this ? I'll rewrite the patch description.
>
> Sorry,
> -Kame
>
>



-- 
Kinds regards,
MinChan Kim
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