Re: [PATCH v6 01/27] x86, mm: Fix page table early allocation offset checking

[Borislav Petkov <bp@alien8.de>]

On Thu, Dec 13, 2012 at 02:01:55PM -0800, Yinghai Lu wrote:
> During debug load kernel above 4G, found one page if is not used in BRK
> and it should be with early page allocation.

What does that mean?

I see that this patch adds a change to not use the page at pgt_buf_top
anymore but why? Is pgt_buf_top the first invalid address we cannot
reserve anymore?

Generally, can we get this whole deal described in a bit more detail for
the mere mortals among us, maybe a short ascii art thing showing what
all those pgt_buf_{start,end,top} mean.

> Fix that checking and also add print out for every allocation from BRK
> page table allocation.
> 
> Signed-off-by: Yinghai Lu <yinghai@kernel.org>
> ---
>  arch/x86/mm/init.c |    4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/mm/init.c b/arch/x86/mm/init.c
> index 6f85de8..c4293cf 100644
> --- a/arch/x86/mm/init.c
> +++ b/arch/x86/mm/init.c
> @@ -47,7 +47,7 @@ __ref void *alloc_low_pages(unsigned int num)
>  						__GFP_ZERO, order);
>  	}
>  
> -	if ((pgt_buf_end + num) >= pgt_buf_top) {
> +	if ((pgt_buf_end + num) > pgt_buf_top) {
>  		unsigned long ret;
>  		if (min_pfn_mapped >= max_pfn_mapped)
>  			panic("alloc_low_page: ran out of memory");
> @@ -61,6 +61,8 @@ __ref void *alloc_low_pages(unsigned int num)
>  	} else {
>  		pfn = pgt_buf_end;
>  		pgt_buf_end += num;
> +		printk(KERN_DEBUG "BRK [%#010lx, %#010lx] PGTABLE\n",

		pr_debug

> +			pfn << PAGE_SHIFT, (pgt_buf_end << PAGE_SHIFT) - 1);
>  	}
>  
>  	for (i = 0; i < num; i++) {
> -- 
> 1.7.10.4

Thanks.

-- 
Regards/Gruss,
    Boris.

Sent from a fat crate under my desk. Formatting is fine.
--