linux-kernel - Re: [PATCH] Create initial stack independent of PAGE

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Date:	Fri, 12 Feb 2010 20:02:16 +1100
From:	Michael Neuling <mikey@...ling.org>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Helge Deller <deller@....de>,
	Americo Wang <xiyou.wangcong@...il.com>,
	Anton Blanchard <anton@...ba.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Create initial stack independent of PAGE_SIZE

In message <20100212161305.73B2.A69D9226@...fujitsu.com> you wrote:
> > Currently we create the initial stack based on the PAGE_SIZE.  This is
> > unnecessary.  
> 
> I don't think this is enough explanation. In past mail, you described
> why page size dependency is harmful. I hope you add it into the patch
> description.

I don't think it's harmful, it's just irrelevant.  Stack size is
independent of page size.  

> IOW, we don't need to change the unnecessary-but-non-harmful behavior.
> 
> > 
> > This creates this initial stack independent of the PAGE_SIZE.
> > 
> > It also bumps up the number of 4k pages allocated from 20 to 32, to
> > align with 64K page systems.
> 
> Why do we need page-aligning? Do you mean this code doesn't works on
> 128K (or more larger) page systems?

If the "random" setting is not a common multiple of the 4k and 64k
pages, they will end up getting aligned differently, hence causing what
we are trying to avoid in the first place with this patch.

I should probably add this as a comment in the code comment?

Mikey

> 
> 
> > 
> > Signed-off-by: Michael Neuling <mikey@...ling.org>
> > ---
> > This is the second half of my original patch. This can be targeted for
> > 2.6.34 as it's just a cleanup.
> > 
> > Tested on PPC64 with 4k and 64k pages.
> > 
> >  fs/exec.c |    4 +---
> >  1 file changed, 1 insertion(+), 3 deletions(-)
> > 
> > Index: linux-2.6-ozlabs/fs/exec.c
> > ===================================================================
> > --- linux-2.6-ozlabs.orig/fs/exec.c
> > +++ linux-2.6-ozlabs/fs/exec.c
> > @@ -554,8 +554,6 @@ static int shift_arg_pages(struct vm_are
> >  	return 0;
> >  }
> >  
> > -#define EXTRA_STACK_VM_PAGES	20	/* random */
> > -
> >  /*
> >   * Finalizes the stack vm_area_struct. The flags and permissions are updat
ed,
> >   * the stack is optionally relocated, and some extra space is added.
> > @@ -630,7 +628,7 @@ int setup_arg_pages(struct linux_binprm 
> >  			goto out_unlock;
> >  	}
> >  
> > -	stack_expand = EXTRA_STACK_VM_PAGES * PAGE_SIZE;
> > +	stack_expand = 131072UL; /* randomly 32*4k (or 2*64k) pages */
> >  	stack_size = vma->vm_end - vma->vm_start;
> >  	/*
> >  	 * Align this down to a page boundary as expand_stack
> 
> 
> 
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