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From: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
To: Wu Fengguang <fengguang.wu@intel.com>
Subject: Re: [PATCH 05/17] writeback: quit throttling when signal pending
Cc: kosaki.motohiro@jp.fujitsu.com, Neil Brown <neilb@suse.de>,
        linux-mm <linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
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        "Li, Shaohua" <shaohua.li@intel.com>
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Date: Tue, 14 Sep 2010 17:44:37 +0900 (JST)
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> On Tue, Sep 14, 2010 at 04:23:56PM +0800, KOSAKI Motohiro wrote:
> > > Subject: writeback: quit throttling when fatal signal pending
> > > From: Wu Fengguang <fengguang.wu@intel.com>
> > > Date: Wed Sep 08 17:40:22 CST 2010
> > > 
> > > This allows quick response to Ctrl-C etc. for impatient users.
> > > 
> > > It mainly helps the rare bdi/global dirty exceeded cases.
> > > In the normal case of not exceeded, it will quit the loop anyway. 
> > > 
> > > CC: Neil Brown <neilb@suse.de>
> > > Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
> > > ---
> > >  mm/page-writeback.c |    3 +++
> > >  1 file changed, 3 insertions(+)
> > > 
> > > --- linux-next.orig/mm/page-writeback.c	2010-09-12 13:25:23.000000000 +0800
> > > +++ linux-next/mm/page-writeback.c	2010-09-13 11:39:33.000000000 +0800
> > > @@ -552,6 +552,9 @@ static void balance_dirty_pages(struct a
> > >  		__set_current_state(TASK_INTERRUPTIBLE);
> > >  		io_schedule_timeout(pause);
> > >  
> > > +		if (fatal_signal_pending(current))
> > > +			break;
> > > +
> > >  check_exceeded:
> > >  		/*
> > >  		 * The bdi thresh is somehow "soft" limit derived from the
> > 
> > I think we need to change callers (e.g. generic_perform_write) too.
> > Otherwise, plenty write + SIGKILL combination easily exceed dirty limit.
> > It mean we can see strange OOM.
> 
> If it's dangerous, we can do without this patch.  

How?


> The users can still
> get quick response in normal case after all.
> 
> However, I suspect the process is guaranteed to exit on
> fatal_signal_pending, so it won't dirty more pages :)

Process exiting is delayed until syscall exiting. So, we exit write syscall
manually if necessary.

Am I missing anything?