linux-kernel - Re: [PATCH 6/7] memcg: add per cgroup writeback pages accounting

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Date:	Mon, 9 Jul 2012 12:14:37 +0800
From:	Fengguang Wu <fengguang.wu@...el.com>
To:	Sha Zhengju <handai.szj@...il.com>
Cc:	linux-mm@...ck.org, cgroups@...r.kernel.org,
	kamezawa.hiroyu@...fujitsu.com, gthelen@...gle.com,
	yinghan@...gle.com, akpm@...ux-foundation.org, mhocko@...e.cz,
	linux-kernel@...r.kernel.org, Sha Zhengju <handai.szj@...bao.com>
Subject: Re: [PATCH 6/7] memcg: add per cgroup writeback pages accounting

On Mon, Jul 09, 2012 at 11:36:11AM +0800, Sha Zhengju wrote:
> On 07/08/2012 10:53 PM, Fengguang Wu wrote:
> >>@@ -2245,7 +2252,10 @@ int test_set_page_writeback(struct page *page)
> >>  {
> >>  	struct address_space *mapping = page_mapping(page);
> >>  	int ret;
> >>+	bool locked;
> >>+	unsigned long flags;
> >>
> >>+	mem_cgroup_begin_update_page_stat(page,&locked,&flags);
> >>  	if (mapping) {
> >>  		struct backing_dev_info *bdi = mapping->backing_dev_info;
> >>  		unsigned long flags;
> >>@@ -2272,6 +2282,8 @@ int test_set_page_writeback(struct page *page)
> >>  	}
> >>  	if (!ret)
> >>  		account_page_writeback(page);
> >>+
> >>+	mem_cgroup_end_update_page_stat(page,&locked,&flags);
> >>  	return ret;
> >>
> >>  }
> >Where is the MEM_CGROUP_STAT_FILE_WRITEBACK increased?
> >
> 
> It's in account_page_writeback().
> 
>  void account_page_writeback(struct page *page)
>  {
> +	mem_cgroup_inc_page_stat(page, MEM_CGROUP_STAT_FILE_WRITEBACK);
>  	inc_zone_page_state(page, NR_WRITEBACK);
>  }

I didn't find that chunk, perhaps it's lost due to rebase..

> There isn't a unified interface to dec/inc writeback accounting, so
> I just follow that.
> Maybe we can rework account_page_writeback() to also account
> dec in?

The current seperate inc/dec paths are fine. It sounds like
over-engineering if going any further.

I'm a bit worried about some 3rd party kernel module to call
account_page_writeback() without mem_cgroup_begin/end_update_page_stat().
Will that lead to serious locking issues, or merely inaccurate
accounting?

Thanks,
Fengguang
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