linux-kernel - Re: FYI: mmap

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Date:	Thu,  8 Jul 2010 20:06:52 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	kosaki.motohiro@...fujitsu.com,
	Michel Lespinasse <walken@...gle.com>,
	linux-mm <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Divyesh Shah <dpshah@...gle.com>, Ingo Molnar <mingo@...e.hu>
Subject: Re: FYI: mmap_sem OOM patch

> On Thu, 2010-07-08 at 19:57 +0900, KOSAKI Motohiro wrote:
> > > On Thu, 2010-07-08 at 03:39 -0700, Michel Lespinasse wrote:
> > > > 
> > > > 
> > > >         One way to fix this is to have T4 wake from the oom queue and return an
> > > >         allocation failure instead of insisting on going oom itself when T1
> > > >         decides to take down the task.
> > > > 
> > > > How would you have T4 figure out the deadlock situation ? T1 is taking down T2, not T4... 
> > > 
> > > If T2 and T4 share a mmap_sem they belong to the same process. OOM takes
> > > down the whole process by sending around signals of sorts (SIGKILL?), so
> > > if T4 gets a fatal signal while it is waiting to enter the oom thingy,
> > > have it abort and return an allocation failure.
> > > 
> > > That alloc failure (along with a pending fatal signal) will very likely
> > > lead to the release of its mmap_sem (if not, there's more things to
> > > cure).
> > > 
> > > At which point the cycle is broken an stuff continues as it was
> > > intended.
> > 
> > Now, I've reread current code. I think mmotm already have this.
> 
> <snip code>
> 
> [ small note on that we really should kill __GFP_NOFAIL, its utter
> deadlock potential ]

I disagree. __GFP_NOFAIL mean this allocation failure can makes really
dangerous result. Instead, OOM-Killer should try to kill next process.
I think.

> > Thought?
> 
> So either its not working or google never tried that code?

Michel?


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