linux-kernel - Re: [BUGFIX][PATCH] memcg: fix race in file

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Date:	Tue, 14 Sep 2010 00:28:30 +0900
From:	Hiroyuki Kamezawa <kamezawa.hiroyuki@...il.com>
To:	balbir@...ux.vnet.ibm.com
Cc:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"nishimura@....nes.nec.co.jp" <nishimura@....nes.nec.co.jp>,
	gthelen@...gle.com,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	stable@...nel.org
Subject: Re: [BUGFIX][PATCH] memcg: fix race in file_mapped accouting flag management

2010/9/13 Balbir Singh <balbir@...ux.vnet.ibm.com>:
> * KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> [2010-09-13 16:08:22]:
>
>>
>> I think this small race is not very critical but it's bug.
>> We have this race since 2.6.34.
>> =
>> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
>>
>> Now. memory cgroup accounts file-mapped by counter and flag.
>> counter is working in the same way with zone_stat but FileMapped flag only
>> exists in memcg (for helping move_account).
>>
>> This flag can be updated wrongly in a case. Assume CPU0 and CPU1
>> and a thread mapping a page on CPU0, another thread unmapping it on CPU1.
>>
>>     CPU0                              CPU1
>>                               rmv rmap (mapcount 1->0)
>>    add rmap (mapcount 0->1)
>>    lock_page_cgroup()
>>    memcg counter+1            (some delay)
>>    set MAPPED FLAG.
>>    unlock_page_cgroup()
>>                               lock_page_cgroup()
>>                               memcg counter-1
>>                               clear MAPPED flag
>>
>> In above sequence, counter is properly updated but FLAG is not.
>> This means that representing a state by a flag which is maintained by
>> counter needs some specail care.
>
> In the situation above who has the PTE lock? Are we not synchronized
> via the PTE lock such that add rmap and rm rmap, will not happen
> simultaneously?
>
In this case, a process for map and one for unmap can be different.

Assume process A maps a file cache and process B not.
While process A unmap a file, process B can map it.
pte lock is no help.

Thanks,
-Kame

Thanks,
-Kame
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