[PATCH] Subject: [Quantal SAUCE] x86/mm: Fix 64bit size of mapping tables

[Stefan Bader]

On 17.08.2012 16:57, Stefan Bader wrote:
> This will also fix some issues with crash-kexec not working anymore
> on 64bit systems. And I hope to get hold of some real humans at
> Plumbers the week after next. But meanwhile...
> 
> -Stefan
> 
> ---
> 
> 
> From d370223977f1e0b4218cb978f480ff4220344eb0 Mon Sep 17 00:00:00 2001
> From: Stefan Bader <stefan.bader at canonical.com>
> Date: Fri, 17 Aug 2012 15:07:27 +0200
> Subject: [PATCH] UBUNTU: SAUCE: (no-up) x86/mm: Fix 64bit size of mapping
>  tables
> 
> commit 722bc6b16771ed80871e1fd81c86d3627dda2ac8
>     x86/mm: Fix the size calculation of mapping tables
> 
> introduced a change to adapt size reserved for the initial mapping
> tables for the first 2/4M range being still 4K. This would looks
> necessary because that range is also covered by MTRRs and it
> would be bad (performance issues at least) to have MTRR ranges
> not aligned with the page boundaries. However there does not seem
> to be a reason to handle 32bit and 64bit differently. Which is exaclty
> what the upstream code (even now) does.
> 
> While trying to get into discussion with upstream about the right
> way to fix this, we should at least make the calculations consistent.
> IOW, for 64bit the first range _is_ 2M pages (or at least not 4K), so
> the additional PTEs are not needed.
> 
> With this applied the size of the initial page tables went from
> 4M to 16K on a VM that had 2G of memory.
> 

BugLink: http://bugs.launchpad.net/bugs/1022561

> Signed-off-by: Stefan Bader <stefan.bader at canonical.com>
> ---
>  arch/x86/mm/init.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/mm/init.c b/arch/x86/mm/init.c
> index bc4e9d8..6219612 100644
> --- a/arch/x86/mm/init.c
> +++ b/arch/x86/mm/init.c
> @@ -60,10 +60,10 @@ static void __init find_early_table_space(struct map_range *mr, unsigned long en
>  		extra = end - ((end>>PMD_SHIFT) << PMD_SHIFT);
>  #ifdef CONFIG_X86_32
>  		extra += PMD_SIZE;
> -#endif
>  		/* The first 2/4M doesn't use large pages. */
>  		if (mr->start < PMD_SIZE)
>  			extra += mr->end - mr->start;
> +#endif
>  
>  		ptes = (extra + PAGE_SIZE - 1) >> PAGE_SHIFT;
>  	} else
> 


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