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Message-ID: <19570.44367.719276.128881@chiark.greenend.org.uk>
Date: Mon, 23 Aug 2010 18:18:07 +0100
To: Peter Zijlstra <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>, Greg KH <gregkh@suse.de>,
        Ian Campbell <ijc@hellion.org.uk>, linux-kernel@vger.kernel.org,
        stable@kernel.org, stable-review@kernel.org, akpm@linux-foundation.org,
        alan@lxorguk.ukuu.org.uk, Jeremy Fitzhardinge <jeremy@goop.org>
Subject: Re: [RFC] mlock/stack guard interaction fixup
In-Reply-To: <1282580751.2605.1997.camel@laptop>
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	<1282470917.11348.891.camel@localhost.localdomain>
	<20100822172548.GB8957@suse.de>
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From: Ian Jackson <ijackson@chiark.greenend.org.uk>
Sender: linux-kernel-owner@vger.kernel.org
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Peter Zijlstra writes ("Re: [RFC] mlock/stack guard interaction fixup"):
> On Mon, 2010-08-23 at 16:42 +0100, ijackson@chiark.greenend.org.uk
> wrote:
> > mlocking the stack is entirely sensible and normal for a real-time
> > program.  Most such programs use mlockall but there is no particular
> > reason why a program that has some more specific requirements should
> > use mlock to lock only a part of the stack.  (Perhaps it has only one
> > real-time thread?) 
> 
> RT apps should pre-allocate and mlock their stack in advance (and
> pre-fault too for the paranoid).

Are you allowed to mlock a stack page which has not yet been faulted
in ?  What effect does it have ?  I wasn't able to find a convincing
de jure answer to this question.

But you seem, like me, to be disagreeing with Linus's assertion that
calling mlock() on the stack is something no sane programs does ?

> mlockall is a very bad interface and should really not be used.

You are directly contradicting the advice in SuS, to which I just gave
a reference.  You're free to do so of course but it might be worth
explaining in a bit more detail why the advice in SuS is wrong.

Ian.
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