[PATCH] Subject: [Quantal SAUCE] x86/mm: Fix 64bit size of mapping tables

Stefan Bader stefan.bader at canonical.com
Fri Aug 17 14:57:09 UTC 2012

This will also fix some issues with crash-kexec not working anymore
on 64bit systems. And I hope to get hold of some real humans at
Plumbers the week after next. But meanwhile...



>From d370223977f1e0b4218cb978f480ff4220344eb0 Mon Sep 17 00:00:00 2001
From: Stefan Bader <stefan.bader at canonical.com>
Date: Fri, 17 Aug 2012 15:07:27 +0200
Subject: [PATCH] UBUNTU: SAUCE: (no-up) x86/mm: Fix 64bit size of mapping

commit 722bc6b16771ed80871e1fd81c86d3627dda2ac8
    x86/mm: Fix the size calculation of mapping tables

introduced a change to adapt size reserved for the initial mapping
tables for the first 2/4M range being still 4K. This would looks
necessary because that range is also covered by MTRRs and it
would be bad (performance issues at least) to have MTRR ranges
not aligned with the page boundaries. However there does not seem
to be a reason to handle 32bit and 64bit differently. Which is exaclty
what the upstream code (even now) does.

While trying to get into discussion with upstream about the right
way to fix this, we should at least make the calculations consistent.
IOW, for 64bit the first range _is_ 2M pages (or at least not 4K), so
the additional PTEs are not needed.

With this applied the size of the initial page tables went from
4M to 16K on a VM that had 2G of memory.

Signed-off-by: Stefan Bader <stefan.bader at canonical.com>
 arch/x86/mm/init.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/mm/init.c b/arch/x86/mm/init.c
index bc4e9d8..6219612 100644
--- a/arch/x86/mm/init.c
+++ b/arch/x86/mm/init.c
@@ -60,10 +60,10 @@ static void __init find_early_table_space(struct map_range *mr, unsigned long en
 		extra = end - ((end>>PMD_SHIFT) << PMD_SHIFT);
 #ifdef CONFIG_X86_32
 		extra += PMD_SIZE;
 		/* The first 2/4M doesn't use large pages. */
 		if (mr->start < PMD_SIZE)
 			extra += mr->end - mr->start;
 		ptes = (extra + PAGE_SIZE - 1) >> PAGE_SHIFT;
 	} else

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