linux-kernel - Re: [PATCH] x86_64: fix page table size

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Date:	Thu, 7 Feb 2008 09:09:30 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Yinghai Lu <Yinghai.Lu@....COM>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86_64: fix page table size

* Yinghai Lu <Yinghai.Lu@....COM> wrote:

> [PATCH] x86_64: fix page table size

> Entering add_active_range(0, 1048576, 67239936) 2 entries of 3200 used
> end_pfn_map = 67239936
> Kernel panic - not syncing: Overlapping early reservations 8000-109fff 
> PGTABLE to 9bc00-9dbff EBDA
> 
> change back the logic. we DO need extra space for pmds when 
> direct_gbpages is not there.

> @@ -370,7 +370,7 @@ static void __init find_early_table_space(unsigned long end)
>  
>  	puds = (end + PUD_SIZE - 1) >> PUD_SHIFT;
>  	tables = round_up(puds * sizeof(pud_t), PAGE_SIZE);
> -	if (direct_gbpages) {
> +	if (!direct_gbpages) {

thanks Yinghai, applied!

I'm wondering why this bug didnt trigger more widely. It seems to me it 
needs some serious amount of RAM to trigger this bug - correct?

btw., it would be nice to have some "lots of RAM simulation" debugging 
code which would just _fake_ a really large e820 map and would in the 
end throw away the 'fake' pages later during bootup. Perhaps tell the 
early allocator to never allocate into these fake areas [via an struct 
e820 entry flag], but all our sizing code and the boot bitmaps, etc. 
would be sized accordingly, as if we had this much RAM - and we'd 
trigger these nuances. We could put this into a new "fakemem=128GB" boot 
option and hence we could boot with fakemem=128GB on a 2GB box and could 
at least hope to be able to boot [with some serious amount of RAM wasted 
on over-sized pagetables, allocator bitmaps and mem_map[]]. Hm?

	Ingo
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