linux-kernel - Re: [PATCH 2/3] binfmt_elf_fdpic: Convert initial stack alignment to arch_align

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Message-ID: <20080801214448.GA13448@linux-sh.org>
Date:	Sat, 2 Aug 2008 06:44:48 +0900
From:	Paul Mundt <lethal@...ux-sh.org>
To:	David Howells <dhowells@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-sh@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] binfmt_elf_fdpic: Convert initial stack alignment to arch_align_stack().

On Fri, Aug 01, 2008 at 03:01:19PM +0100, David Howells wrote:
> Paul Mundt <lethal@...ux-sh.org> wrote:
> 
> > +	 * In some cases (e.g. Hyper-Threading), we want to avoid L1
> > +	 * evictions by the processes running on the same package. One
> > +	 * thing we can do is to shuffle the initial stack for them, so
> > +	 * we give the architecture an opportunity to do so here.
> > +	 */
> >  #ifdef CONFIG_MMU
> > -	sp = bprm->p;
> > +	sp = arch_align_stack(bprm->p);
> >  #else
> > -	sp = mm->start_stack;
> > +	sp = arch_align_stack(mm->start_stack);
> 
> Ummm...  You're calling arch_align_stack() under NOMMU...  Is that really a
> good idea?
> 
Not particularly, no.

> You can't necessarily move the stack pointer without exiting the allocated
> region or shrinking the amount of stack space the executable asked for.  If
> you want to do this sort of thing, you need to tell the memory allocator what
> you're up to - or at the very least allocate some slack.
> 
Yes, that's a good point, and one that probably ought to be documented
alongside the initial alignment. I'll send an updated patch.
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