Re: x86/mm: Limit 2/4M size calculation to x86_32

[Stefan Bader]

On 07/13/2012 06:41 AM, Stefan Bader wrote:
> I was bisecting a problem on 64bit where any attempt to cause a crash kernel to
> boot would hang. The bisect ended up on commit 722bc6b (x86/mm: Fix the size
> calculation of mapping tables) and somehow, looking at the calling function and
> the ranges printed on boot, I think the calculations should only be done in the
> 32bit case.
>
> On 64bit:
> [    0.000000] init_memory_mapping: [mem 0x00000000-0x77e87fff]
> [    0.000000]  [mem 0x00000000-0x77dfffff] page 2M
> [    0.000000]  [mem 0x77e00000-0x77e87fff] page 4k
>
> Attached patch would fix this if you agree with it. Thanks.

Any news on this one? I thought it would be quite simple to check for sanity and 
not wasting memory sounds like a good thing to do. Even though there is plenty 
of it around most of the time. ;)

-Stefan

> -Stefan
>
>
>  From 6b679d1af20656929c0e829f29eed60b0a86a74f Mon Sep 17 00:00:00 2001
> From: Stefan Bader <stefan.bader@xxxxxxxxxxxxx>
> Date: Fri, 13 Jul 2012 15:16:33 +0200
> Subject: [PATCH] x86/mm: Limit 2/4M size calculation to x86_32
>
> commit 722bc6b (x86/mm: Fix the size calculation of mapping tables)
> did modify the extra space calculation for mapping tables in order
> to make up for the first 2/4M memory range using 4K pages.
> However this setup is only used when compiling for 32bit. On 64bit
> there is only the trailing area of 4K pages (which is already added).
>
> The code was already adapted once for things went wrong on a 8TB
> machine (bd2753b x86/mm: Only add extra pages count for the first memory
> range during pre-allocation early page table space), but it looks a bit
> like it currently would overdo things for 64bit.
> I only noticed while bisecting for the reason I could not make a crash
> kernel boot (which ended up on this patch).
>
> Signed-off-by: Stefan Bader <stefan.bader@xxxxxxxxxxxxx>
> Cc: WANG Cong <xiyou.wangcong@xxxxxxxxx>
> Cc: Yinghai Lu <yinghai@xxxxxxxxxx>
> Cc: Tejun Heo <tj@xxxxxxxxxx>
> ---
> diff --git a/arch/x86/mm/init.c b/arch/x86/mm/init.c
> index bc4e9d8..636bbfd 100644
> --- a/arch/x86/mm/init.c
> +++ b/arch/x86/mm/init.c
> @@ -60,10 +60,11 @@ 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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