Date: Mon, 19 Apr 2010 18:11:02 +0200
From: Andrea Arcangeli <aarcange@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Mel Gorman <mel@csn.ul.ie>, r6144 <rainy6144@gmail.com>,
       linux-kernel@vger.kernel.org, Darren Hart <dvhltc@us.ibm.com>,
       tglx <tglx@linutronix.de>, Lee Schermerhorn <lee.schermerhorn@hp.com>
Subject: Re: Process-shared futexes on hugepages puts the kernel in an
 infinite loop in 2.6.32.11; is this fixed now?
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On Mon, Apr 19, 2010 at 05:45:05PM +0200, Peter Zijlstra wrote:
> Wouldn't a longer poison be more recognisable? Also, shouldn't this use
> POISON_POINTER_DELTA?
> 
> Something like:
> 
> #define HUGETBL_POISON	((void *) 0x00300300 + POISON_POINTER_DELTA)
> 
> 0x2e5 isn't that high, I've had actual derefs in that range.

The default at kernel config time sets only 4k as unmapped (I think
it's a very bad default for 64bit archs), so above 4k userland can map
it and you can have actual derefs with 0x00300300 but not with Mel's
preferred <0x1000 address. So the address must be <0x1000.
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