Subject: Re: Change in functionality of futex() system call.
From: Peter Zijlstra <peterz@infradead.org>
To: Eric Dumazet <eric.dumazet@gmail.com>
Cc: David Oliver <david@rgmadvisors.com>, linux-kernel@vger.kernel.org,
        Shawn Bohrer <sbohrer@rgmadvisors.com>,
        Zachary Vonler <zvonler@rgmadvisors.com>,
        KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
        Hugh Dickins <hughd@google.com>, Thomas Gleixner <tglx@linutronix.de>,
        Darren Hart <dvhart@linux.intel.com>, Ingo Molnar <mingo@elte.hu>
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Date: Mon, 06 Jun 2011 18:29:42 +0200
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On Mon, 2011-06-06 at 18:22 +0200, Eric Dumazet wrote:
> Le lundi 06 juin 2011 à 18:16 +0200, Peter Zijlstra a écrit :
> 
> > Hmm, wouldn't that still be susceptible to the zero-page thing if: we
> > create a writable private file map of a sparse file, touch a page and
> > then remap the thing RO?
> > 
> > 
> > 
> 
> Also I am not sure how MAP_PRIVATE could be affected. If we still try a
> RW gup()... It will allocate a page for us, instead of still pointing to
> shared one.
> 
> On previous kernel, the application using read-only mapping could use
> MAP_PRIVATE or MAP_SHARED with same 'behavior'

But by not forcing the COW you get different behaviour depending on when
you call FUTEX_WAIT, surely that's not correct either?
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