linux-kernel - Re: "x86-64, mm: Put early page table high" causes crash on Xen

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Date:	Wed, 2 Mar 2011 15:23:48 +0000
From:	Stefano Stabellini <stefano.stabellini@...citrix.com>
To:	Yinghai Lu <yinghai@...nel.org>
CC:	Stefano Stabellini <Stefano.Stabellini@...citrix.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Jeremy Fitzhardinge <Jeremy.Fitzhardinge@...rix.com>
Subject: Re: "x86-64, mm: Put early page table high" causes crash on Xen

On Tue, 1 Mar 2011, Yinghai Lu wrote:
> We do need map_low_page ( BTW, that name is totally misleading...)
> 
> the reason is we put page_table high and at that time is not under max_pfn_mapped. (aka not mapped).
> 
> So have to use 
> 	adr = early_memremap(phys & PAGE_MASK, PAGE_SIZE);
> to early map it and Read/Write to it.
 
I think I have figured out a way to update the logic of mask_rw_pte to
account for the new way of allocating kernel pagetables.
The appended patch fix the boot crash for me.

---

xen: update mask_rw_pte after kernel page tables init changes

Already existing kernel page table pages can now be mapped using
early_ioremap too so we need to update mask_rw_pte to make sure these
pages are still mapped RO.
We do that by mapping RO all the pages mapped using early_ioremap apart
from the last one that has been allocated because it is not a page table
page yet (it has been hooked into the page tables yet).

Signed-off-by: Stefano Stabellini <stefano.stabellini@...citrix.com>

diff --git a/arch/x86/xen/mmu.c b/arch/x86/xen/mmu.c
index 62192cd..2ff68be 100644
--- a/arch/x86/xen/mmu.c
+++ b/arch/x86/xen/mmu.c
@@ -1440,10 +1440,12 @@ static __init pte_t mask_rw_pte(pte_t *ptep, pte_t pte)
 	/*
 	 * If the new pfn is within the range of the newly allocated
 	 * kernel pagetable, and it isn't being mapped into an
-	 * early_ioremap fixmap slot, make sure it is RO.
+	 * early_ioremap fixmap slot as a freshly allocated page, make sure
+	 * it is RO.
 	 */
-	if (!is_early_ioremap_ptep(ptep) &&
-	    pfn >= pgt_buf_start && pfn < pgt_buf_end)
+	if (((!is_early_ioremap_ptep(ptep) &&
+			pfn >= pgt_buf_start && pfn < pgt_buf_end)) ||
+			(is_early_ioremap_ptep(ptep) && pfn != (pgt_buf_end - 1)))
 		pte = pte_wrprotect(pte);
 
 	return pte;
--
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