Re: [PATCH 04/13] x86, mm: Revert back good_end setting for 64bit

[Stefano Stabellini]

On Sun, 30 Sep 2012, Yinghai Lu wrote:
> After
> 
> | commit 8548c84da2f47e71bbbe300f55edb768492575f7
> | Author: Takashi Iwai <tiwai@xxxxxxx>
> | Date:   Sun Oct 23 23:19:12 2011 +0200
> |
> |    x86: Fix S4 regression
> |
> |    Commit 4b239f458 ("x86-64, mm: Put early page table high") causes a S4
> |    regression since 2.6.39, namely the machine reboots occasionally at S4
> |    resume.  It doesn't happen always, overall rate is about 1/20.  But,
> |    like other bugs, once when this happens, it continues to happen.
> |
> |    This patch fixes the problem by essentially reverting the memory
> |    assignment in the older way.
> 
> Have some page table around 512M again, that will prevent kdump to find 512M
> under 768M.
> 
> We need revert that reverting, so we could put page table high again for 64bit.
> 
> Takashi agreed that S4 regression could be something else.
> 
> 	https://lkml.org/lkml/2012/6/15/182
> 
> Signed-off-by: Yinghai Lu <yinghai@xxxxxxxxxx>
> ---
>  arch/x86/mm/init.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/arch/x86/mm/init.c b/arch/x86/mm/init.c
> index 9f69180..aadb154 100644
> --- a/arch/x86/mm/init.c
> +++ b/arch/x86/mm/init.c
> @@ -76,8 +76,8 @@ static void __init find_early_table_space(struct map_range *mr,
>  #ifdef CONFIG_X86_32
>  	/* for fixmap */
>  	tables += roundup(__end_of_fixed_addresses * sizeof(pte_t), PAGE_SIZE);
> -#endif
>  	good_end = max_pfn_mapped << PAGE_SHIFT;
> +#endif
>  
>  	base = memblock_find_in_range(start, good_end, tables, PAGE_SIZE);
>  	if (!base)

Isn't this going to cause init_memory_mapping to allocate pagetable
pages from memory not yet mapped?
Last time I spoke with HPA and Thomas about this, they seem to agree
that it isn't a very good idea.
Also, it is proven to cause a certain amount of headaches on Xen,
see commit d8aa5ec3382e6a545b8f25178d1e0992d4927f19.
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