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 --- 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; }