Re: [PATCH] x86: mm: accurate the comments for STEP_SIZE_SHIFT macro

[Yinghai Lu <yinghai@kernel.org>]

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On Mon, Mar 18, 2013 at 12:14 PM, H. Peter Anvin <hpa@zytor.com> wrote:

> Instead, try to explain why 5 is the correct value in the current code
> and how it is (or should be!) derived.

initial mapped size is PMD_SIZE, aka 2M.
if we use step_size to be PUD_SIZE aka 1G, as most worse case
that 1G is cross the 1G boundary, and PG_LEVEL_2M is not set,
we will need 1+1+512 pages (aka 2M + 8k) to map 1G range with PTE.
So i picked (30-21)/2 to get 5.

Please check attached patch.

Thanks

Yinghai

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Subject: [PATCH] x86, mm: Add comments for step_size shift

As request by hpa, add comments for why we choose 5 for
step size shift.

Signed-off-by: Yinghai Lu <yinghai@kernel.org>

---
 arch/x86/mm/init.c |   21 ++++++++++++++++++---
 1 file changed, 18 insertions(+), 3 deletions(-)

Index: linux-2.6/arch/x86/mm/init.c
===================================================================
--- linux-2.6.orig/arch/x86/mm/init.c
+++ linux-2.6/arch/x86/mm/init.c
@@ -389,8 +389,23 @@ static unsigned long __init init_range_m
 	return mapped_ram_size;
 }
 
-/* (PUD_SHIFT-PMD_SHIFT)/2 */
-#define STEP_SIZE_SHIFT 5
+static unsigned long __init get_new_step_size(unsigned long step_size)
+{
+	/*
+	 * initial mapped size is PMD_SIZE, aka 2M.
+	 * We can not set step_size to be PUD_SIZE aka 1G yet.
+	 * In worse case, when 1G is cross the 1G boundary, and
+	 * PG_LEVEL_2M is not set, we will need 1+1+512 pages (aka 2M + 8k)
+	 * to map 1G range with PTE. Use 5 as shift for now.
+	 */
+	unsigned long new_step_size = step_size << 5;
+
+	if (new_step_size > step_size)
+		step_size = new_step_size;
+
+	return  step_size;
+}
+
 void __init init_mem_mapping(void)
 {
 	unsigned long end, real_end, start, last_start;
@@ -432,7 +447,7 @@ void __init init_mem_mapping(void)
 		min_pfn_mapped = last_start >> PAGE_SHIFT;
 		/* only increase step_size after big range get mapped */
 		if (new_mapped_ram_size > mapped_ram_size)
-			step_size <<= STEP_SIZE_SHIFT;
+			step_size = get_new_step_size(step_size);
 		mapped_ram_size += new_mapped_ram_size;
 	}