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Message-ID: <20100915034734.GA12264@localhost>
Date: Wed, 15 Sep 2010 11:47:35 +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 11:17:35AM +0800, Neil Brown wrote:
> 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?
Your patch surely breaks the deadlock, however might reintroduce the
old problem too_many_isolated() tried to address..
> 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.
Deadlock means IO/FS tasks (blocked on FS lock) blocking the NOIO/FS
tasks? I think raising the threshold for NOIO/FS would be sufficient
to break the deadlock: The NOIO/FS tasks will be blocked simply
because there are so many NOIO/FS tasks competing with each other.
They do not inherently depend on the release of FS locks to proceed.
The too_many_isolated() was introduced initially to prevent OOM for
some fork-bomb workload, where no IO is involved (so no FS locks). If
removing the congestion wait for NOIO/FS tasks, the OOM may raise
again for the fork-bomb workload.
So I'd suggest to use sufficient high threshold for NOIO/FS, but still
limit the number of concurrent NOIO/FS allocations.
Thanks,
Fengguang
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