[BUGFIX] x86_64: NX bit handling in change_page_attr

["Huang, Ying" <ying.huang@intel.com>]

This patch fixes a bug of change_page_attr/change_page_attr_addr on
Intel x86_64 CPU. After changing page attribute to be executable with
these functions, the page remains un-executable on Intel x86_64
CPU. Because on Intel x86_64 CPU, only if the "NX" bits of all four
level page tables are cleared, the corresponding page is executable
(refer to section 4.13.2 of Intel 64 and IA-32 Architectures Software
Developer's Manual). So, the bug is fixed through clearing the "NX"
bit of PMD when splitting the huge PMD.

Signed-off-by: Huang Ying <ying.huang@intel.com>

---

Index: linux-2.6.23-rc2-mm2/arch/x86_64/mm/pageattr.c
===================================================================
--- linux-2.6.23-rc2-mm2.orig/arch/x86_64/mm/pageattr.c	2007-08-17 12:50:25.000000000 +0800
+++ linux-2.6.23-rc2-mm2/arch/x86_64/mm/pageattr.c	2007-08-17 12:50:48.000000000 +0800
@@ -147,6 +147,7 @@
 			split = split_large_page(address, prot, ref_prot2);
 			if (!split)
 				return -ENOMEM;
+			pgprot_val(ref_prot2) &= ~_PAGE_NX;
 			set_pte(kpte, mk_pte(split, ref_prot2));
 			kpte_page = split;
 		}