linux-kernel - Re: Deadlock possibly caused by too_many

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Date:	Tue, 19 Oct 2010 09:31:42 +1100
From:	Neil Brown <neilb@...e.de>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Wu Fengguang <fengguang.wu@...el.com>,
	Rik van Riel <riel@...hat.com>,
	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>,
	"Li, Shaohua" <shaohua.li@...el.com>
Subject: Re: Deadlock possibly caused by too_many_isolated.

On Mon, 18 Oct 2010 14:58:59 -0700
Andrew Morton <akpm@...ux-foundation.org> wrote:

> On Tue, 19 Oct 2010 00:15:04 +0800
> Wu Fengguang <fengguang.wu@...el.com> wrote:
> 
> > Neil find that if too_many_isolated() returns true while performing
> > direct reclaim we can end up waiting for other threads to complete their
> > direct reclaim.  If those threads are allowed to enter the FS or IO to
> > free memory, but this thread is not, then it is possible that those
> > threads will be waiting on this thread and so we get a circular
> > deadlock.
> > 
> > some task enters direct reclaim with GFP_KERNEL
> >   => too_many_isolated() false
> >     => vmscan and run into dirty pages
> >       => pageout()
> >         => take some FS lock
> > 	  => fs/block code does GFP_NOIO allocation
> > 	    => enter direct reclaim again
> > 	      => too_many_isolated() true
> > 		=> waiting for others to progress, however the other
> > 		   tasks may be circular waiting for the FS lock..
> > 
> > The fix is to let !__GFP_IO and !__GFP_FS direct reclaims enjoy higher
> > priority than normal ones, by honouring them higher throttle threshold.
> > 
> > Now !GFP_IOFS reclaims won't be waiting for GFP_IOFS reclaims to
> > progress. They will be blocked only when there are too many concurrent
> > !GFP_IOFS reclaims, however that's very unlikely because the IO-less
> > direct reclaims is able to progress much more faster, and they won't
> > deadlock each other. The threshold is raised high enough for them, so
> > that there can be sufficient parallel progress of !GFP_IOFS reclaims.
> 
> I'm not sure that this is really a full fix.  Torsten's analysis does
> appear to point at the real bug: raid1 has code paths which allocate
> more than a single element from a mempool without starting IO against
> previous elements.

... point at "a" real bug.

I think there are two bugs here.
The raid1 bug that Torsten mentions is certainly real (and has been around
for an embarrassingly long time).
The bug that I identified in too_many_isolated is also a real bug and can be
triggered without md/raid1 in the mix.
So this is not a 'full fix' for every bug in the kernel :-), but it could
well be a full fix for this particular bug.

NeilBrown

> 
> Giving these allocations the ability to dip further into reserves will
> make occurrence of the bug less likely, but if enough threads all do
> this at the same time, that reserve will be exhausted and we're back to
> square one?
> 
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