linux-kernel - Re: [PATCH mmotm 2.5/4] memcg: disable irq at page cgroup lock (Re: [PATCH -mmotm 3/4] memcg: dirty pages accounting and limiting infrastructure)

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Message-Id: <20100311141300.90b85391.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Thu, 11 Mar 2010 14:13:00 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Daisuke Nishimura <nishimura@....nes.nec.co.jp>
Cc:	balbir@...ux.vnet.ibm.com, linux-mm@...ck.org,
	Andrea Righi <arighi@...eler.com>,
	linux-kernel@...r.kernel.org,
	Trond Myklebust <trond.myklebust@....uio.no>,
	Suleiman Souhlal <suleiman@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	containers@...ts.linux-foundation.org,
	Vivek Goyal <vgoyal@...hat.com>
Subject: Re: [PATCH mmotm 2.5/4] memcg: disable irq at page cgroup lock (Re:
 [PATCH -mmotm 3/4] memcg: dirty pages accounting and limiting
 infrastructure)

On Thu, 11 Mar 2010 13:58:47 +0900
Daisuke Nishimura <nishimura@....nes.nec.co.jp> wrote:
> > I'll consider yet another fix for race in account migration if I can.
> > 
> me too.
> 

How about this ? Assume that the race is very rare.

	1. use trylock when updating statistics.
	   If trylock fails, don't account it.

	2. add PCG_FLAG for all status as

+	PCG_ACCT_FILE_MAPPED, /* page is accounted as file rss*/
+	PCG_ACCT_DIRTY, /* page is dirty */
+	PCG_ACCT_WRITEBACK, /* page is being written back to disk */
+	PCG_ACCT_WRITEBACK_TEMP, /* page is used as temporary buffer for FUSE */
+	PCG_ACCT_UNSTABLE_NFS, /* NFS page not yet committed to the server */

	3. At reducing counter, check PCG_xxx flags by
	TESTCLEARPCGFLAG()

This is similar to an _used_ method of LRU accounting. And We can think this
method's error-range never go too bad number. 

I think this kind of fuzzy accounting is enough for writeback status.
Does anyone need strict accounting ?

Thanks,
-Kame

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