linux-kernel - Re: [2/6] add config option to vmalloc stacks (was: Re: [-mm patch] i386: enable 4k stacks by default)

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Date:	Fri, 4 May 2007 01:35:30 -0400
From:	jfannin@...il.com (Joseph Fannin)
To:	William Lee Irwin III <wli@...omorphy.com>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Christoph Hellwig <hch@...radead.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	David Chinner <dgc@....com>, Zan Lynx <zlynx@....org>,
	Adrian Bunk <bunk@...sta.de>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [2/6] add config option to vmalloc stacks (was: Re: [-mm patch] i386: enable 4k stacks by default)

On Mon, Apr 30, 2007 at 10:43:10AM -0700, William Lee Irwin III wrote:

> +	  Allocates the stack physically discontiguously and from high
> +	  memory. Furthermore an unmapped guard page follows the stack.
> +	  This is not for end-users. It's intended to trigger fatal
> +	  system errors under various forms of stack abuse.

    Why is this not for end-users?  Will it not trigger anything
useful unless set up properly, or is a big performace hit -- and how,
or what?

    All the kernel debug options are underdocumented this way -- I'd
like to have as many of them on as I can without absolutely killing
performance, (or rather, *you* would) -- but I can never tell without
grovelling all over for the info, which... well, I haven't done it
yet, anyway.

    "End-user" is just insufficently defined for anyone compiling
their own kernel.  Could you add a bit more text here describing what
the effect of physically discontiguous high-memory stacks is?  An
additional frobnitz dereference on every badda-bing badda-bang, likely
to double the time it takes to dance the hokey pokey?

   *shrug*  Some of those debug options probably don't get set very
often on kernels that are run for more than to see if it boots.

--
Joseph Fannin
jfannin@...il.com
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