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Old 08-17-2012, 01:07 PM
Stefan Bader
Default UBUNTU: SAUCE: (no-up) x86/mm: Fix 64bit size of mapping tables

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@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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