linux-kernel - Re: Deadlock possibly caused by too_many

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Date:	Wed, 15 Sep 2010 13:17:35 +1000
From:	Neil Brown <neilb@...e.de>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Rik van Riel <riel@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: Deadlock possibly caused by too_many_isolated.

On Wed, 15 Sep 2010 11:06:40 +0800
Wu Fengguang <fengguang.wu@...el.com> wrote:

> On Wed, Sep 15, 2010 at 10:54:54AM +0800, Wu Fengguang wrote:
> > On Wed, Sep 15, 2010 at 10:37:35AM +0800, Wu Fengguang wrote:
> > > On Wed, Sep 15, 2010 at 10:23:34AM +0800, Neil Brown wrote:
> > > > On Tue, 14 Sep 2010 20:30:18 -0400
> > > > Rik van Riel <riel@...hat.com> wrote:
> > > > 
> > > > > On 09/14/2010 07:11 PM, Neil Brown wrote:
> > > > > 
> > > > > > Index: linux-2.6.32-SLE11-SP1/mm/vmscan.c
> > > > > > ===================================================================
> > > > > > --- linux-2.6.32-SLE11-SP1.orig/mm/vmscan.c	2010-09-15 08:37:32.000000000 +1000
> > > > > > +++ linux-2.6.32-SLE11-SP1/mm/vmscan.c	2010-09-15 08:38:57.000000000 +1000
> > > > > > @@ -1106,6 +1106,11 @@ static unsigned long shrink_inactive_lis
> > > > > >   		/* We are about to die and free our memory. Return now. */
> > > > > >   		if (fatal_signal_pending(current))
> > > > > >   			return SWAP_CLUSTER_MAX;
> > > > > > +		if (!(sc->gfp_mask&  __GFP_IO))
> > > > > > +			/* Not allowed to do IO, so mustn't wait
> > > > > > +			 * on processes that might try to
> > > > > > +			 */
> > > > > > +			return SWAP_CLUSTER_MAX;
> > > > > >   	}
> > > > > >
> > > > > >   	/*
> > > > > 
> > > > > Close.  We must also be sure that processes without __GFP_FS
> > > > > set in their gfp_mask do not wait on processes that do have
> > > > > __GFP_FS set.
> > > > > 
> > > > > Considering how many times we've run into a bug like this,
> > > > > I'm kicking myself for not having thought of it :(
> > > > > 
> > > > 
> > > > So maybe this?  I've added the test for __GFP_FS, and moved the test before
> > > > the congestion_wait on the basis that we really want to get back up the stack
> > > > and try the mempool ASAP.
> > > 
> > > The patch may well fail the !__GFP_IO page allocation and then
> > > quickly exhaust the mempool.
> > > 
> > > Another approach may to let too_many_isolated() use much higher
> > > thresholds for !__GFP_IO/FS and lower ones for __GFP_IO/FS. ie. to
> > > allow at least nr2 NOIO/FS tasks to be blocked independent of the
> > > IO/FS ones.  Since NOIO vmscans typically completes fast, it will then
> > > very hard to accumulate enough NOIO processes to be actually blocked.
> > > 
> > > 
> > >                   IO/FS tasks                NOIO/FS tasks           full
> > >                   block here                 block here              LRU size
> > > |-----------------|--------------------------|-----------------------|
> > > |      nr1        |           nr2            |
> > 
> > How about this fix? We may need very high threshold for NOIO/NOFS to
> > prevent possible regressions.
> 
> Plus __GFP_WAIT..
> 
> ---
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 225a759..6a896eb 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1135,6 +1135,7 @@ static int too_many_isolated(struct zone *zone, int file,
>  		struct scan_control *sc)
>  {
>  	unsigned long inactive, isolated;
> +	int ratio;
>  
>  	if (current_is_kswapd())
>  		return 0;
> @@ -1150,7 +1151,15 @@ static int too_many_isolated(struct zone *zone, int file,
>  		isolated = zone_page_state(zone, NR_ISOLATED_ANON);
>  	}
>  
> -	return isolated > inactive;
> +	ratio = 1;
> +	if (!(sc->gfp_mask & (__GFP_FS)))
> +		ratio <<= 1;
> +	if (!(sc->gfp_mask & (__GFP_IO)))
> +		ratio <<= 1;
> +	if (!(sc->gfp_mask & (__GFP_WAIT)))
> +		ratio <<= 1;
> +
> +	return isolated > inactive * ratio;
>  }
>  
>  /*


Are you suggesting this instead of my patch, or as well as my patch?

Because while I think it sounds like a good idea I don't think it actually
removes the chance of a deadlock, just makes it a lot less likely.
So I think your patch combined with my patch would be a good total solution.

Do you agree?

Thanks,
NeilBrown

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