linux-kernel - Re: [RFC] mlock/stack guard interaction fixup

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Date:	Mon, 23 Aug 2010 18:18:07 +0100
From:	Ian Jackson <ijackson@...ark.greenend.org.uk>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Greg KH <gregkh@...e.de>, Ian Campbell <ijc@...lion.org.uk>,
	linux-kernel@...r.kernel.org, stable@...nel.org,
	stable-review@...nel.org, akpm@...ux-foundation.org,
	alan@...rguk.ukuu.org.uk, Jeremy Fitzhardinge <jeremy@...p.org>
Subject: Re: [RFC] mlock/stack guard interaction fixup

Peter Zijlstra writes ("Re: [RFC] mlock/stack guard interaction fixup"):
> On Mon, 2010-08-23 at 16:42 +0100, ijackson@...ark.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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