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

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Date:	Mon, 9 Jul 2012 13:28:13 +0800
From:	Fengguang Wu <fengguang.wu@...el.com>
To:	Sha Zhengju <handai.szj@...il.com>
Cc:	Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	linux-mm@...ck.org, cgroups@...r.kernel.org, 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 01:22:54PM +0800, Sha Zhengju wrote:
> On 07/09/2012 12:18 PM, Kamezawa Hiroyuki wrote:
> >(2012/07/09 13:14), Fengguang Wu wrote:
> >>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?
> >>
> >
> >Ah, Hm. Maybe it's better to add some debug check in
> > mem_cgroup_update_page_stat(). rcu_read_lock_held() or some.
> >
> 
> This also apply to account_page_dirtied()... But as an "range" lock,
> I think it's common
> in current kernel: just as set_page_dirty(), the caller should call
> it under the page lock
> (in most cases) and it's his responsibility to guarantee
> correctness. I can add some
> comments or debug check as reminding but I think i can only do so...

Yeah, it helps to add some brief comment on the locking rule in
account_page_*().

Thanks,
Fengguang
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