linux-kernel - Re: Deadlock possibly caused by too_many

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Message-ID: <20100915025454.GA10230@localhost>
Date:	Wed, 15 Sep 2010 10:54:54 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Neil Brown <neilb@...e.de>
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, 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.

Thanks,
Fengguang
---

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 225a759..5e116cd 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 +1151,9 @@ static int too_many_isolated(struct zone *zone, int file,
 		isolated = zone_page_state(zone, NR_ISOLATED_ANON);
 	}
 
-	return isolated > inactive;
+	ratio = sc->gfp_mask & (__GFP_FS|__GFP_IO) ? 1 : 8;
+
+	return isolated > inactive * ratio;
 }
 
 /*
--
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