From: Larry Woodman <lwoodman@redhat.com>
Date: Tue, 15 Jul 2008 15:31:43 -0400
Subject: [mm] don't use large pages to map the first 2/4MB of mem
Message-id: 1216150303.21188.8.camel@localhost.localdomain
O-Subject: [RHEL5-U3 patch] Backport of don't use large pages to map the first 2/4MB of memory form 2.6.26 to RHEL5-U3
Bugzilla: 455504
RH-Acked-by: Prarit Bhargava <prarit@redhat.com>
RH-Acked-by: Rik van Riel <riel@redhat.com>

The attached patch is a backport of using 4KB pages for the first 2MB of
kernel space for performance reasons.

Fixes BZ 455504.

Gitweb:
http://git.kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=f5c24a7fd0798d636af184cc7032e7e0cb149112
Commit:     f5c24a7fd0798d636af184cc7032e7e0cb149112
Parent:     c9caa02c529d5e113e40cbc77254558fcdfa4215
Author:     Andi Kleen <andi@firstfloor.org>
AuthorDate: Wed Mar 12 03:53:30 2008 +0100
Committer:  Ingo Molnar <mingo@elte.hu>
CommitDate: Thu Apr 17 17:41:30 2008 +0200

    x86: don't use large pages to map the first 2/4MB of memory

    Intel recommends to not use large pages for the first 1MB
    of the physical memory because there are fixed size MTRRs there
    which cause splitups in the TLBs.

    On AMD doing so is also a good idea.

diff --git a/arch/i386/mm/init.c b/arch/i386/mm/init.c
index b9786bd..a7bcfbb 100644
--- a/arch/i386/mm/init.c
+++ b/arch/i386/mm/init.c
@@ -162,8 +162,13 @@ static void __init kernel_physical_mapping_init(pgd_t *pgd_base)
 		for (pmd_idx = 0; pmd_idx < PTRS_PER_PMD && pfn < max_low_pfn; pmd++, pmd_idx++) {
 			unsigned int address = pfn * PAGE_SIZE + PAGE_OFFSET;
 
-			/* Map with big pages if possible, otherwise create normal page tables. */
-			if (cpu_has_pse) {
+			/* Map with big pages if possible, otherwise create normal page tables. 
+			 * Don't use a large page for the first 2/4MB of memory
+			 * because there are often fixed size MTRRs in there
+			 * and overlapping MTRRs into large pages can cause
+			 * slowdowns.
+			 */
+			if (cpu_has_pse && !(pgd_idx == 0 && pmd_idx == 0)) {
 				unsigned int address2 = (pfn + PTRS_PER_PTE - 1) * PAGE_SIZE + PAGE_OFFSET + PAGE_SIZE-1;
 
 				if (is_kernel_text(address) || is_kernel_text(address2))