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

[Jacob Shin]

On Mon, Oct 01, 2012 at 12:00:26PM +0100, Stefano Stabellini wrote:
> 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.
> 

Any comments, thoughts? hpa? Yinghai?

So it seems that during init_memory_mapping Xen needs to modify page table 
bits and the memory where the page tables live needs to be direct mapped at
that time.

Since we now call init_memory_mapping for every E820_RAM range sequencially,
the only way to satisfy Xen is to find_early_page_table_space (good_end needs
to be within memory already mapped at the time) for every init_memory_mapping
call.

What do you think Yinghai?

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