linux-kernel - Re: [PATCH 3/5] writeback: fix dirtied pages accounting on sub-page writes

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Message-ID: <20111122130231.GA18638@localhost>
Date:	Tue, 22 Nov 2011 21:02:31 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Jan Kara <jack@...e.cz>
Cc:	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Christoph Hellwig <hch@....de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/5] writeback: fix dirtied pages accounting on
 sub-page writes

On Tue, Nov 22, 2011 at 08:48:11PM +0800, Jan Kara wrote:
> On Tue 22-11-11 20:30:01, Wu Fengguang wrote:
> > > > @@ -1743,6 +1738,8 @@ void account_page_dirtied(struct page *p
> > > >  		__inc_bdi_stat(mapping->backing_dev_info, BDI_DIRTIED);
> > > >  		task_dirty_inc(current);
> > > >  		task_io_account_write(PAGE_CACHE_SIZE);
> > > > +		current->nr_dirtied++;
> > > > +		__get_cpu_var(bdp_ratelimits)++;
> > >   I think you need preempt_disable() and preempt_enable() pair around
> > > __get_cpu_var(). Otherwise a process could get rescheduled in the middle of
> > > read-modify-write cycle...
> > 
> > Hmm, I'm not worried about it at all, because bdp_ratelimits don't
> > need to be accurate. In normal cases it won't even trigger one single
> > call to balance_dirty_pages().
>   I agree regarding the accuracy. But the CPU can change when the process
> is scheduled again. So you could modify counter of a CPU you are not
> running on. And that can cause bad things...

Will modifying another CPU's per-cpu data lead to more serious
problems than inaccuracy? If not, it would be fine. bdp_ratelimits is
only meant to be a coarse grained safeguard after all :-)

> > btw, account_page_dirtied() is called inside spinlock, will it be
> > sufficient?
>   Currently it is not enough in real-time kernels and when sleeping
> spinlocks work gets merged it won't be enough even in standard kernels...
> And in kernels where spinlock means preemption is disabled
> preempt_enable/disable will be almost for free...

I see, spinlock won't be a general superset of preempt_enable/disable
indeed.

Thanks,
Fengguang
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